Possum means “be able,” “can,” or sometimes “be possible.” Its principal parts are possum, posse, potuī. There is no fourth principal part because the verb has no ordinary participial or passive system.
Most forms become much easier once you see two pieces: an old element pot- meaning “able” plus forms of sum. Before s, pot-s- becomes poss-; before a vowel, pot- remains visible. The perfect system uses potu-.
How the Stems Fit Together
| Environment | Stem/result | Examples |
|---|---|---|
| Before s | pot-s- → poss- | possum, possunt, possim, possem |
| Before a vowel | pot- | potes, potest, poteram, poterō |
| Perfect system | potu- | potuī, potueram, potuerim |
This is a sound pattern, not a choice between spellings. Write possum but potest, never potsum or possest.
Indicative: Complete Paradigm
| Tense | 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. |
|---|---|---|---|---|---|---|
| Present | possum | potes | potest | possumus | potestis | possunt |
| Imperfect | poteram | poterās | poterat | poterāmus | poterātis | poterant |
| Future | poterō | poteris | poterit | poterimus | poteritis | poterunt |
| Perfect | potuī | potuistī | potuit | potuimus | potuistis | potuērunt / potuēre |
| Pluperfect | potueram | potuerās | potuerat | potuerāmus | potuerātis | potuerant |
| Future perfect | potuerō | potueris | potuerit | potuerimus | potueritis | potuerint |
Hoc onus sine auxiliō portāre possum.
I can carry this load without help.
Hodiē venīre nōn possumus; nimis occupātī sumus.
We cannot come today; we are too busy.
Poterās mihi vēritātem dīcere.
You could have told me the truth.
Crās diū manēre poterō.
Tomorrow I shall be able to stay a long time.
The imperfect poteram describes ability or opportunity viewed as continuing in the past. In reproaches such as poterās dīcere, English often says “you could have told,” although Latin uses the imperfect, not the pluperfect.
The perfect potuī presents the opportunity as bounded. In a positive sentence it can imply that the subject actually succeeded, especially with an event that has a clear endpoint, but this is a tendency rather than a rule. Context can contrast ability with performance.
Tandem portam aperīre potuērunt.
At last they managed to open the gate.
Effugere potuit, sed manēre māluit.
He could have escaped, but he preferred to remain.
The second example explicitly cancels the inference that the possible action happened. Do not translate every potuit automatically as “managed to.”
Subjunctive: Complete Paradigm
| Tense | 1st sg. | 2nd sg. | 3rd sg. | 1st pl. | 2nd pl. | 3rd pl. |
|---|---|---|---|---|---|---|
| Present | possim | possīs | possit | possīmus | possītis | possint |
| Imperfect | possem | possēs | posset | possēmus | possētis | possent |
| Perfect | potuerim | potuerīs | potuerit | potuerīmus | potuerītis | potuerint |
| Pluperfect | potuissem | potuissēs | potuisset | potuissēmus | potuissētis | potuissent |
As with sum, the clause supplies the force of the subjunctive. The English translation may use “may,” “might,” “could,” or “could have.”
Nesciō num ad tempus venīre possit.
I do not know whether he can arrive on time.
Sī adessēs, nōs adiuvāre possēs.
If you were here, you could help us.
Utinam plūra facere potuissem!
If only I had been able to do more!
Imperative and Non-Finite Forms
Possum has no normal imperative. Latin commands someone to try, act, or take care rather than commanding the state of ability itself: cōnāre! “try!” or fac ut possīs! “make sure you can!” The future imperative potestō is epigraphically attested but exceptional; it does not supply an ordinary Classical imperative paradigm.
Its Classical non-finite system is also defective:
| Category | Form | Status |
|---|---|---|
| Present infinitive | posse | common |
| Perfect infinitive | potuisse | common |
| Future infinitive | — | no regular form |
| Present participial remnant | potēns, potentis | chiefly an adjective; does not freely fill a verbal participle slot |
| Other participles | — | not formed |
| Gerund, gerundive, supine | — | not formed |
The historically participial adjective potēns means “powerful, capable” and can govern a genitive, but in Classical usage it does not freely fill every missing non-finite slot of possum. For future indirect statement, Latin can use fore ut plus a finite form:
Spērō fore ut crās venīre possīs.
I hope that you will be able to come tomorrow.
Complementary Infinitives
The usual construction is a form of possum + present active or passive infinitive. The subject of possum is also the understood subject of an active infinitive.
Mārcus celeriter currere potest.
Marcus can run quickly.
With a passive infinitive, the thing that can undergo the action is nominative, and possum agrees with it:
Haec sine perīculō fierī nōn possunt.
These things cannot be done without danger.
Compare singular hoc fierī potest “this can be done” with plural haec fierī possunt. English impersonal “it is possible to…” can tempt learners to leave potest singular, but Latin agreement remains visible when there is a nominative subject.
An infinitive can be understood from context:
Quantum potuī, tē adiūvī.
As much as I could, I helped you.
And posse itself can be embedded in indirect statement:
Dīcit sē hoc sine auxiliō facere posse.
He says that he can do this without help.
Here accusative sē is the subject of both posse and, by control, facere. For the broader pattern, see complementary infinitives.
No Productive Compound Family
Unlike sum, possum does not head a productive Classical family such as adsum, absum, and prōsum. Its apparent pieces are already fused inside the single verb. Words such as impotēns “powerless” and potestās “power” are related vocabulary, not compounds that conjugate like possum. This absence is itself important reference information: do not invent verbs such as adpossum.
Common Mistakes
Keeping poss- before every ending
❌ Ea possest venīre.
Incorrect — before the vowel of est, the stem appears as pot-.
✅ Ea potest venīre.
She can come.
Inventing a regular perfect
❌ Herī venīre possuī.
Incorrect — the perfect stem is potu-, not possu-.
✅ Herī venīre potuī.
Yesterday I was able to come.
Giving possum a passive form
❌ Hoc possitur facere.
Incorrect — possum has no passive; the action infinitive can be passive instead.
✅ Hoc fierī potest.
This can be done.
Using an accusative object instead of an infinitive
❌ Possum hunc labōrem.
Incorrect — possum does not mean ‘can’ as a transitive verb governing an accusative object.
✅ Possum hunc labōrem facere.
I can do this work.
Key Takeaways
- Memorize possum, posse, potuī; there is no fourth principal part.
- Poss- appears before s, pot- before a vowel, and potu- throughout the perfect system.
- The verb has no passive or imperative and only two ordinary infinitives.
- It usually governs a complementary infinitive; a passive infinitive lets Latin express “can be done.”
- Potuit can imply successful achievement, but context can explicitly cancel that inference.
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Start learning Latin →Related Topics
- Sum: To BeA2 — A complete reference for sum, including every Classical finite form, non-finite forms, core syntax, and major compounds.
- Eo: To GoA2 — A complete reference for eō, including its alternating stems, contracted perfects, impersonal passive, and major compounds.
- Present Indicative of PossumA1 — Form and use the six present indicative forms of possum, and recognize how its irregular-looking spellings grow from sum.
- Present Subjunctive of Sum and PossumB1 — Form, parse, and use sim and possim, the irregular present subjunctives of Latin's verbs for being and ability.
- Possum, Debeo, and OportetC1 — Distinguish personal ability with possum, subject-centered duty with dēbeō, and impersonal necessity with oportet.
- Complementary InfinitivesA2 — How an infinitive completes the meaning of a modal, volitional, habitual, or phase verb while normally sharing its subject.