Poklopit ću lonac čim voda počne kuhati.

This is how the future tense is commonly formed in standard Croatian when the infinitive comes before the auxiliary.

• infinitive: poklopiti = to cover / put the lid on
• auxiliary: ću = I will

When the infinitive stands directly before ću, an infinitive ending in -ti usually drops the final -i:

• poklopiti ću → poklopit ću

So Poklopit ću lonac means I will cover the pot.

You can also say:

• Ja ću poklopiti lonac
• Lonac ću poklopiti

But ću poklopiti lonac cannot normally stand by itself at the start of the sentence, because ću is a clitic and usually cannot come first.

Also, standard Croatian writes this as two words: poklopit ću, not poklopiću.

Lonac is the direct object, so it is in the accusative singular.

However, lonac is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: lonac = the pot
• accusative: lonac = the pot

That is why the form does not visibly change here.

Croatian often drops subject pronouns when they are not needed.

The form ću already tells you the subject is first person singular: I will.

So:

• Poklopit ću lonac = I will cover the pot
• Ja ću poklopiti lonac = also possible, but ja adds emphasis, like I will cover the pot

In neutral sentences, leaving out ja is very normal.

Čim means as soon as.

It introduces a time clause and emphasizes that one action happens immediately after another:

• Poklopit ću lonac čim voda počne kuhati. = I’ll cover the pot as soon as the water starts boiling.

Compared with kad:

• kad = when
• čim = as soon as

So čim is more specific and immediate.

After conjunctions like čim, Croatian often uses the present tense in the subordinate clause even when the meaning is future.

So instead of saying:

• čim voda će početi... ❌

Croatian says:

• čim voda počne... ✅

This is very normal. English often does something similar:

• I’ll do it when he comes not
• I’ll do it when he will come

So počne is present in form, but in this sentence it refers to a future event.

Počne is the 3rd person singular present tense of the verb početi (to begin / to start).

So:

• ja počnem
• ti počneš
• on/ona/ono počne

Here the subject is voda (water), which is grammatically singular, so we use počne.

After početi (to start / begin), Croatian normally uses an infinitive:

• početi kuhati = to start boiling / to start cooking
• početi raditi = to start working
• početi govoriti = to start speaking

So voda počne kuhati literally follows the pattern water starts to boil.

This is a very standard structure:

• početi + infinitive

Yes. Kuhati can mean both to cook and to boil, depending on context.

With water, voda kuha or voda počne kuhati naturally means that the water is boiling / starting to boil.

So in this sentence:

• voda počne kuhati = the water starts boiling

That is completely normal Croatian usage.

This sentence is a nice example of Croatian aspect.

• poklopiti = perfective
• početi = perfective
• kuhati = imperfective

Why?

1. poklopiti is perfective
It describes a single completed action: putting the lid on the pot.

2. početi is perfective
Starting something is treated as a single point/event: the moment the boiling begins.

3. kuhati is imperfective
Boiling is seen as an ongoing process, so after početi the imperfective infinitive is natural: start boiling, not start to finish boiling.

So the aspect choices are very logical:

• one completed future action
• triggered by the start of an ongoing process

Yes. Croatian word order is fairly flexible.

You can also say:

• Čim voda počne kuhati, poklopit ću lonac.
• Lonac ću poklopiti čim voda počne kuhati.

These all mean roughly the same thing, but the focus changes a little.

A useful thing to remember is that ću is a clitic, so it normally takes second position in its clause:

• poklopit ću
• lonac ću poklopiti
• ja ću poklopiti

That is why word order can move around, but ću still follows clitic-placement rules.