Aspect Choice in the Infinitive

The English infinitive to read is aspect-blind: it tells you nothing about whether the reading is a process or a finished act. The Ukrainian infinitive is not. Every infinitive belongs to one aspect — there is чита́ти (imperfective) and there is прочита́ти (perfective), and you must pick one every time you reach for the infinitive after a modal, a desire verb, or a phase verb. That makes infinitive aspect one of the most relentless meaning-bearing choices in the language: where English says one thing, Ukrainian asks you to also decide whether you mean the activity or its completion. This page sorts the infinitive environments into two kinds — the ones where the rule forces the aspect on you, and the ones where you choose it by meaning — and shows exactly what your choice communicates. For the deeper logic of the phase-verb constraint specifically, see aspect after phase verbs.

The two situations: forced vs free

There are exactly two situations, and the whole skill is telling them apart:

• Forced — the governing verb dictates the infinitive's aspect, and you simply obey. The clearest case: phase verbs demand the imperfective.
• Free — the governing verb permits either aspect, and your choice carries meaning (process vs result, general vs one-off). Modals and desire verbs work this way.

Forced: phase verbs take the imperfective — always

A phase verb names the beginning, middle, or end of an action: почина́ти / поча́ти 'begin', продо́вжувати 'continue', закі́нчувати / закінчи́ти 'finish', перестава́ти / переста́ти 'stop'. The infinitive after any of them is imperfective, with no exceptions — even when the phase verb itself is perfective.

The reason is built into what the aspects mean. To begin, continue, or stop something, you need an action that extends in time — a process with an inside you can enter, stay in, or leave. The perfective presents an action as a single completed point with no interior, so there is nothing to begin into or stop in the middle of. поча́ти + perfective is therefore a contradiction, not just a grammar error. Memorise the hard rule, but lean on the logic so it feels inevitable.

Free: modal and desire verbs — choose by meaning

After хоті́ти 'want', могти́ 'be able', му́сити 'have to', and the impersonals тре́ба 'need to', мо́жна 'may', the infinitive's aspect is yours, and it genuinely changes the message. The split is the familiar one: imperfective = the process / the general activity, perfective = the result / one specific completed act.

These are not interchangeable. The first wants the experience of reading; the second wants the finished book. The same fork runs through every modal:

Notice the default pull of single planned acts: "buy bread", "make a call", "send the email" are normally one-and-done errands, so they take the perfective (купи́ти, подзвони́ти, наді́слати) unless you specifically mean a repeated or ongoing version.

Verbs that lean perfective: забу́ти, всти́гнути, зуміти

A small set of governing verbs pulls strongly toward the perfective infinitive, because they are about whether a single discrete act gets accomplished: забу́ти 'forget (to do)', всти́гнути 'manage in time / have time to', зуміти 'manage to', спромогти́ся 'contrive to'. They frame the infinitive as one accomplishment that did or did not happen.

These aren't an absolute rule like the phase verbs — забу́ти can take an imperfective when you mean a habitual neglect (забу́в полива́ти кві́ти 'kept forgetting to water the flowers') — but the strong default is perfective, and you should reach for it first.

Negated necessity leans imperfective

When you negate a necessity or permission — не тре́ба 'no need to', не мо́жна 'must not', не ва́рто 'not worth', не слід 'shouldn't' — the infinitive strongly prefers the imperfective. Telling someone not to engage in an action targets the activity as such, which is the imperfective's viewpoint. This rhymes with the negative imperative (see aspect in the imperative).

A perfective can return after a negated modal when the sense flips to a single accidental outcome to avoid — Диви́сь, щоб не впа́сти ('mind you don't fall') — but for ordinary "no need / not allowed to do X", the imperfective is the safe default.

Negated 'forget' flips the aspect

A subtle but useful contrast: не забу́ти ('don't forget to') keeps the perfective, because you are reminding someone to accomplish one act — whereas the negated necessity verbs above lean imperfective. The two negations point in opposite directions.

So the negation rule is not "negation always means imperfective". It depends on the verb: не тре́ба / не ва́рто + imperfective (don't engage in the activity), but не забу́дь + perfective (do remember to complete this one act).

Source-language comparison

For an English speaker, the whole challenge is that the infinitive has an aspect at all. English to read is a single, aspect-free word; Ukrainian forces you to also encode process-vs-result every single time. Two habits fix most errors. First, on phase verbs, learn the rule as a fact and never reach for a perfective infinitive after поча́ти / переста́ти / продо́вжувати — even though English happily says "started to finish it". Second, on modals and desire verbs, pause and ask whether you mean the activity (imperfective: хо́чу чита́ти) or the finished result (perfective: хо́чу прочита́ти). English carries that distinction in extra words ("read" vs "read it through"); Ukrainian carries it in the verb stem.

For a Russian speaker, the architecture is identical — phase verbs force the imperfective (начать + imperfective), modals choose by meaning, забыть/успеть lean perfective, не надо/не стоит lean imperfective. Relearn the Ukrainian forms (поча́ти / почина́ти, переста́ти, продо́вжувати, всти́гнути, тре́ба, ва́рто) and their stress; the rules port across unchanged.

Common Mistakes

Key Takeaways

• Every Ukrainian infinitive carries aspect — you choose чита́ти vs прочита́ти every time, encoding process vs result where English encodes nothing.
• Phase verbs (поча́ти, переста́ти, продо́вжувати, закінчи́ти) force the imperfective infinitive: поча́в учи́ти, never *поча́в ви́вчити. This is a hard rule grounded in meaning — you can't begin or stop a single completed point.
• Modal / desire verbs (хоті́ти, могти́, тре́ба, мо́жна) let you choose by meaning: imperfective = process / general (хо́чу чита́ти), perfective = result / one act (хо́чу прочита́ти); single planned errands default to perfective (тре́ба купи́ти хлі́ба).
• забу́ти, всти́гнути, зуміти lean perfective (one accomplished act): забу́в зачини́ти, всти́гну купи́ти.
• Negated necessity (не тре́ба, не ва́рто, не мо́жна) leans imperfective (не тре́ба хвилюва́тися) — but не забу́дь keeps the perfective (не забу́дь зателефонува́ти), because it points at completing one act.