Kad film bude gotov, vratit ćemo se kući.

Because Croatian often uses future II in subordinate time clauses introduced by words like kad when one future action must happen before another.

• Kad film bude gotov = When the film is finished / when the film is over
• vratit ćemo se kući = we will return home

So the idea is: first the film finishes, then we go home.

Why not će biti gotov?
In Croatian, će biti is the normal future form in a main clause, but after kad in this kind of sentence, bude is the usual and correct choice.

Compare:

• Film će biti gotov u deset. = The film will be finished at ten.
• Kad film bude gotov, vratit ćemo se kući. = When the film is finished, we’ll return home.

Yes. Kad and kada both mean when.

• kad is shorter and very common in everyday speech
• kada is a bit more formal or emphatic

So:

• Kad film bude gotov...
• Kada film bude gotov...

Both are correct.

Here gotov means finished, over, or done.

It is an adjective, and it agrees with film:

• film is masculine singular
• so the adjective is gotov (masculine singular)

Other forms would be:

• knjiga je gotova = the book is finished
• jelo je gotovo = the meal is ready / finished

In this sentence, film bude gotov means the movie is over / finished.

In standard Croatian spelling, the future tense with verbs in -ti is written like this when the auxiliary comes after the verb:

• vratit ćemo
• ići ćemo
• radit ćemo

So vratiti ćemo is not standard.

What happens is:

• infinitive: vratiti
• future auxiliary: ćemo
• in writing, the final -ti drops before ćemo
• result: vratit ćemo

You can also avoid this structure by putting the auxiliary first:

• Mi ćemo se vratiti kući.

That is also correct.

Because the verb is vratiti se, which means to return.

So in Croatian, vratiti by itself usually means to return something:

• Vratio sam knjigu. = I returned the book.

But vratiti se means to return / come back:

• Vratili smo se kući. = We returned home.

So here se is not optional. It is part of the verb meaning.

Because se is a clitic, and Croatian clitics have special word-order rules. They usually come in an early position in the clause, but not at the very beginning.

In this sentence, the natural standard order is:

• vratit ćemo se kući

You can also say:

• Mi ćemo se vratiti kući.

But:

• Se vratit ćemo kući is not correct.

For learners, the safest thing is to memorize vratiti se and common future patterns like:

• vratit ćemo se
• ćemo se vratiti

Because kući is the normal form used in the expression meaning home after verbs of going or returning:

• ići kući = to go home
• doći kući = to come home
• vratiti se kući = to return home

This is a very common fixed usage in Croatian.

It does not mean exactly the same thing as u kuću:

• kući = home
• u kuću = into the house

So in your sentence, kući is the natural choice.

Because Kad film bude gotov is a subordinate time clause, and here it comes before the main clause.

So Croatian writes:

• Kad film bude gotov, vratit ćemo se kući.

The comma separates:

• the time clause: Kad film bude gotov
• the main clause: vratit ćemo se kući

This is very normal punctuation in Croatian.

Yes, absolutely. That is a very natural alternative.

Compare:

• Kad film bude gotov... = When the film is finished / over
• Kad film završi... = When the film ends

Both are correct and common.

The difference is slight:

• bude gotov focuses on the state of being finished
• završi focuses on the event of ending

In everyday use, both work well here.

It is future I, the normal future tense.

Structure:

• verb stem: vratit-
• auxiliary: ćemo = we will
• plus se because the verb is vratiti se

So:

• vratit ćemo se = we will return

This is the main action of the sentence.

Yes. You can also put the main clause first:

• Vratit ćemo se kući kad film bude gotov.

This means the same thing: We’ll return home when the movie is over.

When the kad clause comes first, the sentence often feels a little more like setting the scene first:

• Kad film bude gotov, vratit ćemo se kući.

When the main clause comes first, it can sound a bit more direct:

• Vratit ćemo se kući kad film bude gotov.

Both are correct.

A rough breakdown is:

• Kad = when
• film = film / movie
• bude = future form of biti used in this kind of clause
• gotov = finished / over
• vratit ćemo se = we will return
• kući = home

So very literally:

• When the movie is finished, we will return home.

In natural English, that would usually be:

• When the movie is over, we’ll go back home.
• When the movie is finished, we’ll return home.