The in- prefix is French's main tool for negating adjectives. Possible becomes impossible; capable becomes incapable; visible becomes invisible. The prefix is inherited from Latin (where the same morpheme produced infinitus, immortalis, illegitimus, irrationalis) and English has inherited it twice over — once through Latin loanwords (invisible, immortal) and once through French (illegal, irresponsible).
For English speakers, this is one of the most rewarding word-formation patterns to internalize: hundreds of French negative adjectives are essentially identical to their English counterparts. The challenge is mastering the spelling rule that distributes the prefix into four forms — in-, im-, il-, ir- — depending on the first letter of the base word.
The assimilation rule
The prefix has a fixed pattern. Its final consonant matches the first consonant of the base word, producing what linguists call assimilation. The four forms are distributed phonologically:
| Form | Used before | Examples |
|---|---|---|
| in- | most consonants and vowels | inutile, incompréhensible, indispensable, injuste, invisible |
| im- | m, p, b | impossible, immortel, immangeable, imbattable, impatient |
| il- | l | illégal, illogique, illisible, illimité, illettré |
| ir- | r | irréel, irrégulier, irréparable, irrésistible, irrespectueux |
The logic is purely phonetic: French speakers find /in.m/, /in.p/, /in.l/, /in.r/ awkward at the morpheme boundary, so the n of the prefix assimilates to the following sound. The result is a doubled consonant in writing (immortel, i*llégal, i**rréel*) that is pronounced as a single sustained sound, not as two separate consonants.
Pronunciation note: the n of in- is pronounced as a nasal vowel when followed by a consonant other than another n: inutile /i.ny.til/ (no nasal — the n attaches to the following vowel), but injuste /ɛ̃.ʒyst/ (nasal in-). The forms im-, il-, ir- never have the nasal pronunciation — the doubled consonant simply blocks it. Impossible is /ɛ̃.pɔ.sibl/ but immortel is /i.mɔʁ.tɛl/ (no nasal, because im- is followed by m).
C'est impossible à finir avant demain.
It's impossible to finish before tomorrow.
Sa réaction était complètement irrationnelle.
His reaction was completely irrational.
Cette écriture est illisible, je ne peux rien déchiffrer.
This handwriting is illegible — I can't make anything out.
Le bruit est devenu insupportable, je vais sortir.
The noise has become unbearable — I'm going to go out.
In- before a vowel
When the base word begins with a vowel, the prefix is in- but the n is pronounced — there is no nasal vowel. The /i/ stays oral and the /n/ links to the following vowel: inutile /i.ny.til/, inactif /i.nak.tif/, inégal /i.ne.ɡal/. Compare injuste /ɛ̃.ʒyst/ (consonant follows, nasal /ɛ̃/) — the same prefix, two pronunciations decided by what comes next.
| Base adjective | Negated form | Meaning |
|---|---|---|
| utile (useful) | inutile | useless |
| actif (active) | inactif | inactive |
| égal (equal) | inégal | unequal |
| habité (inhabited) | inhabité | uninhabited |
| humain (human) | inhumain | inhuman |
| évitable (avoidable) | inévitable | unavoidable |
| oubliable (forgettable) | inoubliable | unforgettable |
| attendu (expected) | inattendu | unexpected |
C'était une soirée inoubliable, merci de m'avoir invité.
It was an unforgettable evening — thanks for inviting me.
Cette région est presque inhabitée depuis vingt ans.
This region has been almost uninhabited for twenty years.
Sa visite était complètement inattendue.
His visit was completely unexpected.
In- before consonants (other than m, p, b, l, r)
The straightforward case. The prefix attaches as in- before c, d, f, g, j, n, q, s, t, v, and z, with no spelling change. The pronunciation is nasalized: /ɛ̃-/.
| Base adjective | Negated form | Meaning |
|---|---|---|
| capable (capable) | incapable | incapable |
| compétent (competent) | incompétent | incompetent |
| complet (complete) | incomplet | incomplete |
| connu (known) | inconnu | unknown |
| compréhensible (understandable) | incompréhensible | incomprehensible |
| cohérent (coherent) | incohérent | incoherent |
| croyable (believable) | incroyable | unbelievable, incredible |
| certain (certain) | incertain | uncertain |
| direct (direct) | indirect | indirect |
| défini (defined) | indéfini | indefinite |
| discret (discreet) | indiscret | indiscreet, nosy |
| dépendant (dependent) | indépendant | independent |
| fidèle (faithful) | infidèle | unfaithful |
| juste (fair, just) | injuste | unfair |
| nombrable (countable) | innombrable | countless |
| suffisant (sufficient) | insuffisant | insufficient |
| visible (visible) | invisible | invisible |
| variable (variable) | invariable | invariable, unchanging |
C'est incroyable, je n'arrive pas à y croire !
It's incredible — I can't believe it!
Sa réponse était totalement incohérente.
His answer was totally incoherent.
Le menu propose un nombre innombrable de desserts.
The menu offers a countless number of desserts.
C'est injuste, je n'ai rien fait !
It's unfair — I didn't do anything!
Im- before m, p, b
When the base word begins with m, p, or b, the prefix becomes im-. The doubling of m in immortel or the mp sequence in impossible signals the morpheme boundary.
| Base adjective | Negated form | Meaning |
|---|---|---|
| possible (possible) | impossible | impossible |
| patient (patient) | impatient | impatient |
| parfait (perfect) | imparfait | imperfect |
| pair (paired, even) | impair | odd (number), uneven |
| poli (polite) | impoli | impolite |
| prévisible (predictable) | imprévisible | unpredictable |
| prudent (prudent) | imprudent | reckless |
| productif (productive) | improductif | unproductive |
| propre (clean / own) | impropre | improper, unfit |
| mortel (mortal) | immortel | immortal |
| mangeable (edible) | immangeable | inedible |
| mature | immature | immature |
| buvable (drinkable) | imbuvable | undrinkable |
| battable (beatable) | imbattable | unbeatable |
Ce café est imbuvable, il y a trop de sucre.
This coffee is undrinkable — there's too much sugar.
Son comportement est imprévisible, on ne sait jamais à quoi s'attendre.
His behavior is unpredictable — you never know what to expect.
C'est impossible à finir aujourd'hui, soyons réalistes.
It's impossible to finish today — let's be realistic.
Sept est un nombre impair, neuf aussi.
Seven is an odd number, so is nine.
Il- before l
When the base word begins with l, the prefix becomes il- and the ll is pronounced as a single /l/.
| Base adjective | Negated form | Meaning |
|---|---|---|
| légal (legal) | illégal | illegal |
| légitime (legitimate) | illégitime | illegitimate |
| logique (logical) | illogique | illogical |
| lisible (legible) | illisible | illegible |
| limité (limited) | illimité | unlimited |
| licite (lawful) | illicite | unlawful |
| lettré (lettered, literate) | illettré | illiterate |
Le téléchargement de ce contenu est illégal en France.
Downloading this content is illegal in France.
Mon forfait téléphone est illimité, je peux appeler autant que je veux.
My phone plan is unlimited — I can call as much as I want.
Son raisonnement est complètement illogique.
His reasoning is completely illogical.
Ir- before r
When the base word begins with r, the prefix becomes ir-. The rr is pronounced as a single trilled or guttural /r/.
| Base adjective | Negated form | Meaning |
|---|---|---|
| réel (real) | irréel | unreal |
| régulier (regular) | irrégulier | irregular |
| rationnel (rational) | irrationnel | irrational |
| recevable (admissible) | irrecevable | inadmissible |
| réparable (repairable) | irréparable | irreparable |
| résistible (resistible) | irrésistible | irresistible |
| respectueux (respectful) | irrespectueux | disrespectful |
| responsable (responsible) | irresponsable | irresponsible |
| réfléchi (thought-out) | irréfléchi | thoughtless |
| récupérable (recoverable) | irrécupérable | unrecoverable |
Le verbe 'aller' est irrégulier à presque toutes les personnes.
The verb 'aller' is irregular in almost every person.
Ce dessert est irrésistible, je n'ai pas pu m'empêcher d'en reprendre.
This dessert is irresistible — I couldn't help having seconds.
Il est complètement irresponsable, il ne pense jamais aux conséquences.
He's completely irresponsible — he never thinks about consequences.
In- + -able: the combined productive pattern
One of the most productive patterns in modern French combines two morphemes: the in- / im- / il- / ir- negative prefix and the -able / -ible suffix (capable of being V-ed). The result is "not capable of being V-ed."
| Verb | -able form | In- negation |
|---|---|---|
| imaginer | imaginable | inimaginable |
| croire | croyable | incroyable |
| contester | contestable | incontestable |
| éviter | évitable | inévitable |
| tolérer | tolérable | intolérable |
| supporter | supportable | insupportable |
| oublier | oubliable | inoubliable |
| discuter | discutable | indiscutable |
| prévoir | prévisible | imprévisible |
| réparer | réparable | irréparable |
| réfuter | réfutable | irréfutable |
Sa générosité est incroyable, il a tout donné à l'association.
His generosity is incredible — he gave everything to the charity.
Le bruit est devenu insupportable, on a dû fermer les fenêtres.
The noise became unbearable — we had to close the windows.
C'était un moment inoubliable de notre voyage.
It was an unforgettable moment of our trip.
This pattern is so productive that you can derive almost any in- + -able combination on the fly. If a learner needs to say "uninvitable," non-invitable would be the safer choice, but ininvitable is at least parseable.
When in- attaches to nouns
The prefix occasionally attaches to nouns, producing the negation of a noun. This is less common than the adjective pattern, and the resulting nouns tend to be abstract.
| Base noun | In- negation | Meaning |
|---|---|---|
| action | inaction | inaction |
| justice | injustice | injustice |
| égalité | inégalité | inequality |
| compétence | incompétence | incompetence |
| certitude | incertitude | uncertainty |
| discrétion | indiscrétion | indiscretion |
| mobilité | immobilité | immobility |
| pertinence | impertinence | impertinence |
| légalité | illégalité | illegality |
| régularité | irrégularité | irregularity |
Les inégalités sociales restent un problème majeur.
Social inequalities remain a major problem.
L'incertitude économique pèse sur les décisions d'investissement.
Economic uncertainty weighs on investment decisions.
When in- does NOT negate
A trap: not every word starting with in- contains the negative prefix. Many Latin-derived French words begin with in- meaning "into," "in," or just inherit in- as part of a Latin root. Context tells you which.
| Word | Apparent base | Actual meaning |
|---|---|---|
| inviter | "un-viter"? | invite (from Latin invitare; no negation) |
| insister | "un-sister"? | insist (from Latin insistere, "stand on") |
| inscrire | "un-write"? | register, inscribe (in + scribere = write in) |
| imposer | "un-poser"? | impose (in + poser = put on) |
| immigrer | "un-migrate"? | immigrate (in + migrare = move into) |
| illuminer | "un-lighten"? | illuminate (in + lumen = into light) |
| irriguer | "un-rigate"? | irrigate (in + rigare = bring water to) |
In these cases, in- / im- / il- / ir- derives from the Latin preposition meaning "into" rather than from the Latin negative prefix. The two prefixes are historically distinct but converge in modern spelling. Learn the negative-prefix pattern by recognizing the base word: if the base is a modern French adjective with the opposite meaning (possible / impossible, légal / illégal), the prefix is negative; if the base is a noun root with no contrastive adjective, the prefix is more likely the "into" sense.
Comparison with non-
French has a second productive negation tool: the prefix-like element non-, attached with a hyphen to a noun, adjective, or participle. It is more analytic and often more recent — many non- formations are alternatives to in- forms that do not exist or feel awkward.
| In- form (if exists) | Non- form | Use |
|---|---|---|
| incomplet | non-complet | in- is standard |
| — | non-fumeur | no in- form exists |
| — | non-violence | technical term |
| — | non-citoyen | administrative |
| — | non-respect | standard formulation |
| — | non-conformiste | standard |
| — | non-dit | "what is left unsaid" |
C'est un restaurant non-fumeur, c'est plus agréable.
It's a non-smoking restaurant — it's more pleasant.
Les non-dits dans cette famille pèsent lourd.
The unspoken truths in this family weigh heavily.
The non- prefix is more productive for new formations; in- is the established form for the classical adjectives. When you need to negate a modern compound or technical term, non- is usually the safer choice.
Source-language note: English vs French
English and French share the in- / im- / il- / ir- prefix system almost completely, because English borrowed it directly from Latin and through French. The mapping is one of the most reliable in the language:
- possible / impossible → possible / impossible
- legal / illegal → légal / illégal
- responsible / irresponsible → responsable / irresponsable
- patient / impatient → patient / impatient
- visible / invisible → visible / invisible
The few traps:
- English sometimes uses un- where French uses in-. English uncertain vs French incertain; English unhappy (no in- form) vs French malheureux (no in- form either — different strategy). The systems mostly overlap, but the lexical inventory differs.
- A few false friends: infamous in English means "very famous for bad reasons"; French infâme (with the same root) means "vile, despicable." The English shift to "very famous" is a semantic drift.
- English allows in-
- -ed (uninvited, unexpected, unwanted)
For practical purposes: when you encounter a French adjective whose root is also an English word, try the in- / im- / il- / ir- negation rule first. You will be right four times out of five.
Common Mistakes
Mistake 1: Using in- before m, p, b, l, or r.
❌ C'est inpossible.
Before p, the prefix becomes im-. Spell impossible.
✅ C'est impossible.
It's impossible.
Mistake 2: Using im- or il- with the wrong base letter.
❌ Sa conduite est imrationnelle.
Before r, the prefix becomes ir-: irrationnelle (with double r).
✅ Sa conduite est irrationnelle.
His behavior is irrational.
Mistake 3: Reading in- as negative when it is not.
❌ Je suis invité ce soir — c'est triste.
Invité means 'invited,' not 'un-vited.' The in- here is the Latin 'into' prefix, not the negative.
✅ Je suis invité ce soir, je suis content.
I'm invited tonight — I'm happy.
Mistake 4: Failing to double the consonant in writing.
❌ Cette écriture est ilisible.
The doubled l is required: illisible, not *ilisible.
✅ Cette écriture est illisible.
This handwriting is illegible.
Mistake 5: Inventing in- forms where French uses a different strategy.
❌ Il est inintelligent.
The form inintelligent technically exists but is awkward; native French prefers 'peu intelligent' or 'pas intelligent.'
✅ Il n'est pas très intelligent.
He isn't very intelligent.
Mistake 6: Confusing in- (negation) with non- (also negation).
❌ C'est un non-possible.
When an in- form exists (impossible), use it. Non- is reserved for forms where in- doesn't work or sounds odd.
✅ C'est impossible.
It's impossible.
Key takeaways
- The negative prefix has four forms distributed phonologically: in- (default and before vowels), im- (before m, p, b), il- (before l), ir- (before r).
- The doubled consonant in immortel, illégal, irréel is a single sustained sound in pronunciation, not two consonants.
- The prefix attaches mainly to adjectives (possible/impossible), abstract nouns (justice/injustice), and the productive -able class (croyable/incroyable, évitable/inévitable).
- Not every word starting with in- is negated: inviter, inscrire, imposer, illuminer contain a different in- meaning "into."
- For new formations and technical terms, French often prefers non- (non-fumeur, non-violence) over in-. Learn both and pick the right one for the register.
- English and French share this prefix system almost completely. When in doubt, try the in- / im- / il- / ir- form first — the spelling and meaning usually line up with English in-, im-, il-, ir-, or un-.
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