One of the most efficient ways to negate in Brazilian Portuguese is to never write não at all — instead, you fold the negation into the word itself with a prefix. Possível becomes impossível; honesto becomes desonesto. This page covers the four productive negative prefixes — in- (with its variants), des-, a-/an-, and anti- — plus the assimilation rules that make in- reshape itself before certain consonants. Getting that assimilation exactly right is what separates a learner's guess from a correctly spelled word.
Why prefixes, not "não"?
A prefix negates a concept, while não negates a clause. Compare Ele não é honesto ("He isn't honest") with Ele é desonesto ("He is dishonest"). The second predicates a positive trait — dishonesty — rather than merely denying honesty. Prefixed negatives also slot into noun phrases where a clausal não can't go: uma decisão impossível, um gesto desnecessário. English does exactly the same with un-, in-, dis- and non-, so the idea transfers cleanly; the work is in learning which prefix and what spelling.
1. The "in-" family (and its assimilation)
in- is the Latin-derived negative prefix corresponding to English in-/un- (incapaz, inútil, infeliz). Its defining feature is that the final n assimilates to the following sound, changing its spelling. These rules are not optional or stylistic — they are fixed orthography.
| Before… | Prefix becomes | Examples |
|---|---|---|
| p, b | im- | impossível, impaciente, imbatível, imberbe |
| l | i- (n drops, l doubles → il-) | ilegal, ilegível, ilimitado, ilógico |
| r | ir- (n → r) | irreal, irregular, irresponsável, irracional |
| m | im- (n → m; the root's m merges, so one m is written) | imaterial, imóvel, imortal |
| vowels & other consonants | in- (unchanged) | incapaz, inútil, infeliz, incerto, inadequado |
The mnemonic is mechanical: the prefix matches the next sound. Before the lip-sounds p and b, the tongue can't hold an n, so it becomes the lip-sound m → im-. Before l and r, the n assimilates completely, doubling the consonant: in+legal → ilegal, in+real → irreal. Everywhere else it stays in-.
Pra mim, isso é totalmente impossível.
For me, that's completely impossible.
Estacionar aqui é ilegal.
Parking here is illegal.
A resposta dele foi irresponsável.
His answer was irresponsible.
Que letra mais ilegível!
What unreadable handwriting!
One honest caveat: not every word starting with in- is negated. Incrível (incredible), informação (information) and instalar (to install) just happen to begin with those letters — the in- is part of the root, not a negator. Likewise imenso (immense) is not "not-mense". Context and meaning decide; there is no shortcut except recognising the base word.
2. "des-" — the most productive negator
If in- is bookish and fixed (mostly attached to Latinate adjectives), des- is alive and productive: Brazilians attach it to verbs and nouns freely. It carries two related senses:
- Reversal / undoing (the bigger group): fazer → desfazer (undo), ligar → desligar (switch off), montar → desmontar (disassemble), abotoar → desabotoar (unbutton).
- Negation / privation: honesto → desonesto (dishonest), necessário → desnecessário (unnecessary), confiar → desconfiar (to distrust), emprego → desemprego (unemployment).
Pode desligar o ar-condicionado? Tô com frio.
Can you turn off the air conditioning? I'm cold.
Acho desnecessário discutir isso agora.
I think it's unnecessary to argue about this now.
Ele desfez as malas assim que chegou.
He unpacked the suitcases as soon as he arrived.
Desconfio que ela já sabia de tudo.
I suspect she already knew everything.
The reversal sense maps neatly onto English un-/dis- ("undo", "disassemble"), and the privative sense onto dis-/un- ("dishonest", "unnecessary"). Because des- is so productive, you can sometimes coin a word on the fly and be understood — destravar (unjam), desinstalar (uninstall) — but check a dictionary before assuming a coinage is standard.
3. "a-/an-" — Greek privative
The Greek-derived a- (and an- before a vowel) means "lacking / without", attached mostly to learned or technical adjectives.
| Before… | Form | Examples |
|---|---|---|
| consonant | a- | amoral, ateu, atípico, apolítico |
| vowel | an- | anormal, analfabeto, anestesia, anaeróbio |
A decisão dele foi completamente amoral.
His decision was completely amoral.
Esse comportamento é anormal pra ele.
This behaviour is abnormal for him.
Mind the difference between amoral ("having no moral sense", a- = lacking) and imoral ("immoral", in- = against morals). The prefixes encode different relationships, and Brazilians keep them distinct — a useful illustration that the choice of negative prefix carries meaning.
4. "anti-", "contra-", and the minor players
anti- = "against / opposed to" (antissocial, antivírus, anti-inflamatório). Post-AO90 spelling doubles the s when anti- meets an s-initial root (antissocial, antisséptico) — and likewise doubles the r before an r-root (antirrugas) — but takes a hyphen only before h or the same vowel the prefix ends in, i.e. i (anti-inflamatório, anti-herói). Before other vowels it joins solid: antiaéreo, antibacteriano.
Ele é meio antissocial, prefere ficar em casa.
He's a bit antisocial; he'd rather stay home.
Comprei um antivírus novo pro computador.
I bought a new antivirus for the computer.
contra- = "counter / against" (contradizer, contraproducente, contramão). And a couple of rarer negators round things off: sem- in fixed compounds (sem-vergonha = shameless rascal, sem-teto = homeless person) and mal- (malsucedido = unsuccessful, mal-educado = rude).
Brigar agora seria totalmente contraproducente.
Arguing now would be totally counterproductive.
Que menino sem-vergonha!
What a shameless kid!
Common Mistakes
❌ Isso é inpossível.
Incorrect — before p, 'in-' must assimilate to 'im-'.
✅ Isso é impossível.
That's impossible.
The single most common spelling slip. Before p or b, always im-.
❌ Estacionar aqui é inlegal.
Incorrect — before l, 'in-' becomes 'i-' and the l doubles: 'ilegal'.
✅ Estacionar aqui é ilegal.
Parking here is illegal.
❌ A resposta foi inresponsável.
Incorrect — before r, 'in-' becomes 'ir-': 'irresponsável'.
✅ A resposta foi irresponsável.
The answer was irresponsible.
❌ Ele agiu de forma amoral, contra todas as regras morais.
Wrong prefix — 'against morals' is 'imoral'; 'amoral' means 'without moral sense'.
✅ Ele agiu de forma imoral, contra todas as regras.
He acted immorally, against all the rules.
Choose the prefix by meaning: a- = lacking, im-/in- = contrary to.
❌ Ele é antisocial.
Incorrect — post-AO90, 'anti-' + s-root doubles the s: 'antissocial'.
✅ Ele é antissocial.
He's antisocial.
Key Takeaways
- in- assimilates: im- before p/b, i- before l (→ ilegal), ir- before r (→ irreal), in- elsewhere. These spellings are obligatory.
- des- is the most productive prefix: reversal (desligar, desfazer) and negation (desonesto, desnecessário).
- a-/an- (Greek privative) means "lacking"; keep amoral (no morals) distinct from imoral (against morals).
- anti- doubles s/r before s-/r-roots (antissocial, antirrugas) and hyphenates only before h or i (anti-inflamatório, anti-herói).
- Not every word in in- is a negation — incrível, informação and imenso carry the letters by coincidence.
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Start learning Portuguese→Related Topics
- Common PrefixesB1 — The productive Brazilian Portuguese prefixes — negation, repetition, intensity, and position — most of which map directly onto English, plus the post-AO90 hyphenation rules.
- Negation: OverviewA1 — How Brazilian Portuguese says no — 'não' before the verb, obligatory negative concord, the emphatic 'não...não' tail, and a map of the whole negation system.
- Negative Words at Sentence Start (No 'Não' Needed)A2 — All the ways Brazilian Portuguese expresses negation without using 'não' — fronted negative words, 'sem', 'nem', prefixes, and lexical negatives.
- Word Formation: OverviewB1 — How Brazilian Portuguese builds words from roots, prefixes, and suffixes — and why learning the morphemes multiplies your vocabulary instead of merely adding to it.