Aspect with Phase and Modal Verbs

When one verb governs another in the infinitive, the governing verb can constrain which aspect the infinitive may take. Two groups matter here. Phase verbs — početi "begin", prestati "stop", nastaviti "continue" — impose a hard grammatical rule: their infinitive complement must be imperfective. Modal verbs — morati "must", moći "can", htjeti "want", smjeti "may", trebati "need" — impose no such restriction; they take either aspect, and the choice carries the usual process-versus-result meaning. The phase-verb rule is the headline, because it is a genuine constraint (not a stylistic preference) and because English's flexible "start to read / start reading" hides it completely.

Phase verbs require an imperfective infinitive

A phase verb names the beginning, continuation, or end of an action. Logically, you can only begin, continue, or stop a process — something that unfolds over time. You cannot "begin" a completed whole, because a completed whole has no internal duration to enter into. Croatian encodes this directly: phase verbs grammatically block the perfective and demand the imperfective infinitive.

(Note that the phase verb itself comes in an aspect pair — početi pf / počinjati impf — and chooses its own aspect normally. The rule is about the infinitive it governs, which must be imperfective regardless of the phase verb's own aspect.)

In every case the infinitive is the imperfective member: čitati not pročitati, pričati not ispričati, padati not pasti. Swapping in the perfective produces an ungrammatical sentence, not merely an awkward one.

Why English hides this rule

English lets you say "start to read" or "start reading" with no aspect marking at all, and it happily allows "I started to read the whole book" — a phrasing whose Croatian literal equivalent (*početi pročitati) is impossible. Because English's gerund and infinitive are aspect-neutral, an English speaker has no instinct that "begin" should reject a "completed" complement. Croatian makes the underlying logic visible: you begin a process, so the complement must be the process-viewing (imperfective) member.

This also explains a translation pattern. English "I'll finish reading the book" maps to Croatian in two different ways depending on what you mean:

The second sentence packs the completion into the perfective verb directly, with no phase verb at all — which is often the more natural Croatian way to say "I'll get the book read".

Modal verbs take either aspect

Modals are different. Morati, moći, htjeti, smjeti, trebati and the like impose no aspect restriction on their infinitive. You choose the aspect for the usual reason — a single completed result versus a general or ongoing action — and the modal simply layers obligation, ability, or volition on top.

Set two modal sentences side by side and you see the aspect doing exactly what it always does:

Putting the two rules together

The cleanest way to keep these straight is to ask what the governing verb is about. A phase verb is about the unfolding of an action — begin / continue / stop — so it can only attach to a process: imperfective, always. A modal verb is about your relation to an action — must / can / want — which is indifferent to whether the action is a one-off result or an ongoing process, so it leaves the aspect choice open to you.

This stacked example shows both rules at once: the modal morati freely takes the phase verb početi, and the phase verb in turn forces the imperfective učiti.

Common Mistakes

Key Takeaways

• Phase verbs (početi, prestati, nastaviti, završiti) take an imperfective infinitive — always. It is a hard grammatical constraint: you begin/continue/stop a process, not a completed whole.
• "I started reading" is therefore uniquely Počeo sam čitati; *početi pročitati is ungrammatical.
• The phase verb chooses its own aspect normally; the rule applies only to the infinitive it governs.
• Modal verbs (morati, moći, htjeti, smjeti, trebati) take either aspect, with the usual process-vs-result meaning: Moram napisati pismo (one result) vs Moram pisati svaki dan (a habit).
• English's aspect-neutral "start to read / start reading" hides this split — make the imperfective-after-phase-verb rule an automatic reflex.