Sum is Latin’s verb “be,” but it also expresses existence, location, possession, and many idioms. Its principal parts are sum, esse, fuī, futūrus. The change from s-/es- in the present system to fu- in the perfect and future participle is inherited irregularity, not a spelling accident.
Learn the principal parts as one unit. They prevent invented forms such as essīvī: the perfect “I was” is fuī.
Principal Parts and Stems
| Principal part | What it supplies | Example |
|---|---|---|
| sum | present-system finite forms | sum, eram, erō, sim, essem |
| esse | present infinitive | esse |
| fuī | perfect-system active forms | fuī, fueram, fuerō, fuerim |
| futūrus | future participle and future infinitive | futūrus esse |
Sum is a copular and intransitive verb. It has no passive voice: forms such as “was loved” use sum only as the auxiliary beside a passive participle (amātus est), not as a passive form of sum itself.
Indicative: Complete Paradigm
| Tense | 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. |
|---|---|---|---|---|---|---|
| Present | sum | es | est | sumus | estis | sunt |
| Imperfect | eram | erās | erat | erāmus | erātis | erant |
| Future | erō | eris | erit | erimus | eritis | erunt |
| Perfect | fuī | fuistī | fuit | fuimus | fuistis | fuērunt / fuēre |
| Pluperfect | fueram | fuerās | fuerat | fuerāmus | fuerātis | fuerant |
| Future perfect | fuerō | fueris | fuerit | fuerimus | fueritis | fuerint |
The perfect fuī may mean “I was” or “I have been,” according to context. The imperfect eram presents a continuing or background state; the perfect presents the state as a bounded whole or marks a change of narrative stage.
Nunc domī sum, sed herī Rōmae fuī.
I am at home now, but yesterday I was in Rome.
Cum puer eram, prope mare habitābam.
When I was a boy, I lived near the sea.
Crās in forō erimus.
Tomorrow we shall be in the forum.
Subjunctive: Complete Paradigm
| Tense | 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. |
|---|---|---|---|---|---|---|
| Present | sim | sīs | sit | sīmus | sītis | sint |
| Imperfect | essem | essēs | esset | essēmus | essētis | essent |
| Perfect | fuerim | fuerīs | fuerit | fuerīmus | fuerītis | fuerint |
| Pluperfect | fuissem | fuissēs | fuisset | fuissēmus | fuissētis | fuissent |
The subjunctive does not have one fixed English translation. It can mark purpose, result, reported thought, a wish, a possibility, or a condition. The clause decides whether English needs “may be,” “would be,” “were,” or another expression.
Utinam omnēs salvī sint!
May everyone be safe!
Sī adessēs, laetior essem.
If you were here, I would be happier.
Nesciō ubi fuerit.
I do not know where he has been.
The (literary) forms forem, forēs, foret, forent can replace certain imperfect-subjunctive forms; see forem, fores, and forent.
Imperatives
| Tense/register | 2nd sg. | 3rd sg. | 2nd pl. | 3rd pl. |
|---|---|---|---|---|
| Present, ordinary | es | — | este | — |
| Future, formal/legal | estō | estō | estōte | suntō |
The future imperatives are (formal/archaic) in Classical prose, especially at home in laws, instructions, and solemn prescriptions. Their “future” name reflects an order intended to apply afterward, not an English command with “will.”
Este parātī; nūntius mox adveniet.
Be ready; the messenger will arrive soon.
Iūdex iūstus estō.
The judge shall be just. (formal/legal)
Non-Finite Forms and Missing Forms
| Category | Form | Note |
|---|---|---|
| Present infinitive | esse | “to be” |
| Perfect infinitive | fuisse | “to have been” |
| Future infinitive | futūrum esse | agrees in gender and number: futūram, futūrōs, futūrās |
| Alternative future infinitive | fore | indeclinable, often with ut + subjunctive |
| Future active participle | futūrus, -a, -um | “going to be, about to be” |
Classical sum has no ordinary present participle, perfect participle, gerund, gerundive, or supine. The form ēns “being” belongs mainly to later philosophical and grammatical Latin and should not be inserted into a Classical paradigm as though Cicero used it freely.
Dīcit sē parātum esse.
He says that he is ready.
Dīxit sē Rōmae fuisse.
He said that he had been in Rome.
Spērō nōs mox līberōs futūrōs esse.
I hope that we shall soon be free.
Core Syntax
With a predicate noun or adjective, both subject and predicate normally stand in the nominative. Latin does not copy English word order mechanically.
Lūcia medica est et frāter eius poēta est.
Lucia is a doctor, and her brother is a poet.
Here Lūcia and medica refer to the same person and are both nominative. The predicate noun agrees naturally in gender where the noun has a corresponding feminine form.
Possession is often expressed with a dative possessor plus a nominative thing possessed—literally, “there is X to me.”
Mihi duo librī sunt.
I have two books.
The double dative combines a dative of purpose with a dative of reference:
Haec epistula nōbīs magnō auxiliō fuit.
This letter was a great help to us.
Compounds of Sum
Most compounds preserve recognizable forms of sum, but the prefix may change shape. They have their own meanings and case patterns.
| Verb | Meaning | Typical construction |
|---|---|---|
| absum, abesse, āfuī | be absent/distant | ab
|
| adsum, adesse, adfuī / affuī | be present, assist | often dative |
| dēsum, deesse, dēfuī | be lacking, fail | dative |
| intersum, interesse, interfuī | be among; take part; concern | dative for “take part”; impersonal interest with genitive or possessive adjective |
| obsum, obesse, obfuī | hinder, harm | dative |
| praesum, praeesse, praefuī | be in charge of | dative |
| prōsum, prōdesse, prōfuī | benefit | dative; prōd- before a vowel |
| supersum, superesse, superfuī | remain, survive | dative when “survive someone” |
Legātus exercituī praeerat, sed ab urbe aberat.
The envoy was in command of the army but was away from the city.
For full formation and syntax, see sum and its compounds.
Common Mistakes
Inventing a regular perfect stem
❌ Herī Rōmae essīvī.
Incorrect — sum does not build its perfect by adding -ivi to esse.
✅ Herī Rōmae fuī.
Yesterday I was in Rome.
Giving a predicate adjective the accusative
❌ Mārcus laetum est.
Incorrect — the predicate adjective agrees with nominative Marcus.
✅ Mārcus laetus est.
Marcus is happy.
Assuming English “have” is the only option
⚠️ Ego habeō duo frātrēs.
Valid and idiomatic Latin, but it does not illustrate the possessive-dative construction with sum.
✅ Mihi duo frātrēs sunt.
I have two brothers.
Inventing a passive of sum
❌ Urbs essitur magna.
Incorrect — sum has no passive paradigm.
✅ Urbs magna est.
The city is large.
Key Takeaways
- Memorize sum, esse, fuī, futūrus; the stem changes are real and highly frequent.
- Sum has indicative, subjunctive, and imperative forms, but no passive voice.
- Predicate nouns and adjectives are normally nominative; possession and several idioms use the dative.
- The Classical non-finite inventory is small and has important gaps.
- Compounds keep much of sum’s irregular morphology but must be learned with their own meanings and case government.
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Start learning Latin →Related Topics
- Possum: To Be AbleA2 — A complete reference for possum, its assimilated present forms, perfect stem, infinitive complements, and defective paradigm.
- Sum and Its CompoundsA2 — Read the complete system of sum and form its common prefixed compounds from a small set of irregular but reusable stems.
- Present Indicative of SumA1 — Master sum, es, est, sumus, estis, and sunt in identity, description, location, existence, and basic possession.
- Predicate NominativesA1 — Use and recognize nominative nouns that identify, classify, rename, or characterize a nominative subject.
- Dative of PossessionA2 — Understand how sum plus a dative expresses having, availability, and personal association in Classical Latin.
- Forem, Fores, and ForentC2 — Recognize the old imperfect-subjunctive forms of sum, their Classical uses, and their relationship to essem and fore.