Kad profesorica počne predavanje, svi zašute.

Kad means when.

In this sentence, kad profesorica počne predavanje means when the professor starts the lecture.

Kad and kada usually mean the same thing. The difference is mostly style:

• kad = more common in everyday speech
• kada = often a bit more formal or emphatic

So you could also say:

• Kada profesorica počne predavanje, svi zašute.

with basically the same meaning.

Because počne is from a perfective verb: početi = to begin / to start as a completed starting event.

Croatian aspect matters a lot here:

• početi → perfective → focuses on the moment something starts
• počinjati / početi (počinje as imperfective present in usage from počinjati/početi pair depending analysis) → imperfective → focuses more on the ongoing process or repeated activity

In this sentence, the important thing is the moment the lecture begins, so počne is the natural choice.

Compare:

• Kad profesorica počne predavanje...
  = When the professor starts the lecture...
• Kad profesorica počinje predavanje...
  sounds much less natural for this meaning, because it points toward the process of starting rather than the completed onset.

Yes, počne is grammatically a present-tense form.

It is:

• 3rd person singular
• of the verb početi
• in the perfective aspect

The tricky part is that in Croatian, a perfective present often does not mean a normal ongoing present. Instead, it is often used for:

• future-related meaning after words like kad (when)
• general repeated situations
• event sequences

So kad profesorica počne predavanje can mean:

• when the professor starts the lecture
• whenever the professor starts the lecture

depending on context.

This is very normal in Croatian.

Because predavanje is the direct object of počne.

The verb početi can take a direct object, so predavanje is in the accusative singular.

Here the noun is predavanje = lecture.

For neuter nouns like this one, the nominative singular and accusative singular are often the same, so the form does not visibly change:

• nominative: predavanje
• accusative: predavanje

So even though the form looks the same, its role here is accusative: it is the thing being started.

Zašute means become silent, fall silent, or go quiet.

It comes from the perfective verb zašutjeti, which focuses on the change into silence.

That is different from šute, which means are silent / keep quiet and describes the state itself.

So the contrast is:

• zašute = they go quiet
• šute = they are quiet

In your sentence, the meaning is about the reaction at that moment: the lecture begins, and then everyone falls silent. That is why zašute fits so well.

Svi means all or everyone, depending on context.

Here it means everyone.

Why plural? Because Croatian often expresses everyone with a plural form:

• svi = all people / everyone

It also agrees with the verb in the plural:

• svi zašute = everyone falls silent

You could also use svatko:

• Kad profesorica počne predavanje, svatko zašuti.

That is grammatically possible and means roughly the same thing, but svi zašute sounds very natural and common.

In Croatian, the masculine plural is the default form for a mixed group or for people in general.

So:

• svi = all / everyone (general or mixed group)
• sve = all (for an all-female group)

That means:

• svi zašute = everyone falls silent / all of them fall silent
• sve zašute = all of them fall silent, if the group is entirely female

In your sentence, svi is just the normal general form.

Because the sentence begins with a subordinate clause:

• Kad profesorica počne predavanje = subordinate time clause
• svi zašute = main clause

When the subordinate clause comes first, Croatian normally separates it from the main clause with a comma:

• Kad profesorica počne predavanje, svi zašute.

If you reverse the order, you will often see:

• Svi zašute kad profesorica počne predavanje.

In that order, there is often no comma before kad.

Most naturally, it sounds like a general repeated situation:

• Whenever the professor starts the lecture, everyone falls silent.

That is because the sentence uses perfective present forms in a way that often describes a typical reaction or repeated pattern.

If you wanted to make it sound more clearly like one specific future occasion, Croatian often uses future in the main clause:

• Kad profesorica počne predavanje, svi će zašutjeti.

That means more clearly:

• When the professor starts the lecture, everyone will fall silent.

So your original sentence is very natural for a general rule, habit, or repeated scene.

Yes. Croatian word order is fairly flexible.

You can also say:

• Svi zašute kad profesorica počne predavanje.

This has essentially the same meaning.

The version with kad first may sound a little more like setting the scene first:

• When the professor starts the lecture, everyone falls silent.

The version with svi zašute first may put a little more focus on the result:

• Everyone falls silent when the professor starts the lecture.

Both are normal.

Yes, both verbs are perfective:

• počne ← početi
• zašute ← zašutjeti

That makes sense because the sentence is about two single transitions:

1. the lecture starts
2. everyone goes silent

Perfective verbs are very common when Croatian describes event boundaries like this.

If you used imperfective verbs instead, the meaning would shift toward an ongoing state or process rather than those transitions.

So the perfective pair gives the sentence a very neat sequence:

• first the professor starts the lecture
• then everyone falls silent