Possum, Debeo, and Oportet | Latin Grammar

Possum, dēbeō, and oportet all combine naturally with infinitives, but they organize the situation differently. Possum venīre says “I can come”: the subject has ability or opportunity. Dēbeō venīre says “I ought to come / I am obliged to come”: a duty rests on the subject. Mē venīre oportet says “I ought to come”: the necessity is presented impersonally, and “me” is accusative as the infinitive's subject.

English auxiliaries such as “can,” “should,” “ought,” and “must” hide these structural differences. Latin makes them visible in agreement and case. Parse the finite verb and its infinitival unit before selecting an English modal. Unless marked otherwise, the examples below are (neutral Classical prose) and use pedagogical macrons.

The core contrast

Verb	Principal parts	Person affected	Core force
possum	possum, posse, potuī	nominative subject	ability, opportunity, possibility
dēbeō	dēbeō, dēbēre, dēbuī, dēbitum	nominative subject	debt, duty, expectation
oportet	oportet, oportēre, oportuit	normally accusative subject of infinitive	impersonal propriety or necessity

Hodiē venīre possum, sed domī manēre dēbeō.

I can come today, but I ought to remain at home.

Mē ante noctem domum redīre oportet.

I ought to return home before nightfall.

The first sentence has an unexpressed nominative “I” in both finite personal verbs. In the second, oportet remains third-person singular, while accusative mē belongs to the infinitive redīre. It is not the direct object of “it behooves” in a way that can be freely replaced by nominative ego.

💡

Use agreement as the fastest diagnostic. Possumus and dēbēmus are first-person plural because “we” is their subject; nōs ... oportet keeps third-person singular oportet and places “us” in the accusative.

Possum: ability and possibility

Possum fuses pot- with forms of sum: possum, potes, potest; possumus, potestis, possunt. Imperfect and future use pot- (poteram, poterō), while the perfect system uses potu- (potuī, potueram, potuerō). It has active morphology and normally takes a same-subject complementary infinitive.

Lūcia hās litterās sine auxiliō legere potest.

Lucia can read this letter without help.

Postquam medicus vēnit, puer tandem dormīre potuit.

After the doctor arrived, the boy was finally able to sleep.

The imperfect often presents continuing capacity or opportunity; the perfect presents the ability as a bounded fact and may imply successful achievement in a suitable context.

Portam aperīre poterant, sed nōluērunt.

They could open the gate, but they refused.

Portam tandem aperīre potuērunt.

They finally managed to open the gate.

This aspectual contrast is a tendency, not a guarantee that potuit always means “managed.” Context still decides whether an available ability was exercised. For the full irregular paradigm, see Possum.

Debeo: from debt to duty

The concrete sense of dēbeō is “owe.” A direct object names what is owed, and a dative can name the creditor.

Tibi decem dēnāriōs dēbeō; crās pecūniam reddam.

I owe you ten denarii; I shall repay the money tomorrow.

With an infinitive, that debt metaphor broadens into obligation, expectation, or what is due from someone. The nominative subject remains personally responsible.

Iūdex utramque partem audīre dēbet.

The judge ought to hear both sides.

Amīcī in adversīs temporibus fidēlēs esse dēbent.

Friends should be loyal in difficult times.

The force of dēbeō varies. In rules or moral argument it can approach “must”; in advice it may be “should”; in prediction it can express what is expected. It does not possess a special modal conjugation: dēbet is present indicative, dēbēbat imperfect, dēbuit perfect, and dēbeat present subjunctive.

Past dēbēbat or dēbuit can imply an obligation that was not fulfilled, especially when the context contrasts duty with what happened.

Herī venīre dēbēbās, sed tē frūstrā exspectāvimus.

You were supposed to come yesterday, but we waited for you in vain.

The nonfulfillment comes from sed tē frūstrā exspectāvimus, not from the imperfect alone. Without context, venīre dēbēbās states a past obligation and leaves fulfillment open.

Oportet: impersonal necessity or propriety

Modal oportet is impersonal and normally stays third-person singular: oportet “it is proper/necessary,” oportēbat “it was proper/necessary,” oportēbit “it will be necessary,” oportuit “it was necessary / ought to have happened.” A person who performs the infinitive appears in the accusative.

Nōs vēritātem dīcere oportet, etiam sī molesta est.

We ought to tell the truth, even if it is unpleasant.

Tē prius rem tōtam cognōscere oportuit.

You ought to have learned all the facts first.

The present infinitive after past oportuit is not a tense error. It marks the action required at the time of the past necessity; oportuit locates that necessity. English often uses “ought to have learned,” whose perfect infinitive has no one-to-one structural match here.

Oportet can also take a subjunctive clause without ut when a full finite proposition is required. This compact construction continues an older independent hortatory or optative pattern and is especially at home in deliberative and philosophical prose (formal).

Oportet cōnsul lēgibus pāreat.

The consul ought to obey the laws.

Cicero uses accusative tē with a passive infinitive and imperfect oportēbat in a deliberately severe accusation (formal oratory):

Ad mortem te, Catilina, duci iussu consulis iam pridem oportebat.

You, Catiline, ought long ago to have been led to death by order of the consul. (Cicero, In Catilinam 1.2; source spelling reproduced without added macrons.)

The verb stays singular. Tē is accusative subject of passive dūcī; oportēbat locates the claimed necessity in past time.

Personal versus impersonal passive infinitives

The contrast becomes especially clear when the required or possible action is passive.

Latin	Structure	Translation
Epistula mittī potest.	nominative epistula controls personal potest	The letter can be sent.
Epistula mittī dēbet.	nominative epistula controls personal dēbet	The letter ought to be sent.
Epistulam mittī oportet.	accusative epistulam is subject of mittī; oportet is impersonal	The letter ought to be sent.

Hae litterae statim legī possunt et legī dēbent.

These letters can be read at once and ought to be read.

Hās litterās statim legī oportet.

These letters ought to be read at once.

The English translations conceal the case contrast. Latin plural possunt and dēbent agree with nominative hae litterae; impersonal singular oportet governs an infinitival clause whose subject is accusative hās litterās.

A decision guide

Ask these questions in order:

Is the point capacity, opportunity, or possibility? Use possum.
Is a duty, debt, or expectation assigned to a personal subject? Use dēbeō.
Is the proposition evaluated impersonally as proper or necessary? Use oportet.
Is a specific object presented as something that must be acted on? Consider the passive periphrastic: epistula mihi mittenda est “I must send the letter.”

Hoc facere possum, hoc facere dēbeō, hoc mē facere oportet.

I can do this; I ought to do this; it is proper that I do this.

The three clauses form a minimal contrast. The action does not change; the speaker changes its modal framing and, with oportet, its syntax.

Negation and scope

Negation requires attention to meaning, not just word order. Nōn possum venīre denies ability: “I cannot come.” Nōn dēbeō venīre can deny an obligation (“I am not required to come”) or, in context, convey that coming is contrary to duty (“I ought not come”). Mē nōn venīre oportet most transparently places the negative inside the required proposition: “it is proper for me not to come.” Classical word order and discourse emphasis allow variation, so do not claim that the position of nōn alone always resolves its semantic scope.

💡

English “must not” and “do not have to” differ sharply. Latin can make the distinction through context, an explicitly negated infinitive, or alternatives such as nōn necesse est. Never assume that adding nōn to one modal gives a single automatic English result.

Common Mistakes

1. Making the person with oportet nominative

❌ Ego nunc discēdere oportet.

Incorrect — oportet is impersonal, and the infinitive's subject must be accusative.

✅ Mē nunc discēdere oportet.

I ought to leave now.

2. Making oportet agree with a plural person

❌ Nōs crās venīre oportēmus.

Incorrect — oportet does not acquire first-person plural agreement.

✅ Nōs ad conventum crās venīre oportet.

We ought to come to the meeting tomorrow.

3. Treating English can, should, and must as tense labels

❌ Herī venīre possum.

Incorrect for a completed past ability — possum is present.

✅ Herī venīre potuī.

Yesterday I was able to come.

4. Adding ut before a same-subject complementary infinitive

❌ Iūdex dēbet ut utramque partem audiat.

Incorrect as the ordinary personal dēbeō construction.

✅ Bonus iūdex utramque partem patienter audīre dēbet.

A good judge ought to hear both sides patiently.

5. Using nominative with an impersonal passive-infinitive clause

❌ Epistula mittī oportet.

Incorrect in the ordinary impersonal analysis — epistula must be accusative as the subject of mittī.

✅ Epistulam mittī oportet.

The letter ought to be sent.

Key Takeaways

Possum assigns ability to a nominative subject; dēbeō assigns debt, duty, or expectation to a nominative subject; oportet evaluates a proposition impersonally and normally places the infinitive's subject in the accusative. Their finite forms locate the modal relation in time, while the infinitive supplies its content. Translate the whole structure only after checking agreement, case, and the intended kind of modality.

Now practice Latin

Reading grammar gets you part of the way. The exercises are where it sticks — free, no signup needed.

Start learning Latin →