Ako ostane vremena, napravit ćemo vlak od plastelina i malu kuću.

Because Croatian often uses the genitive after expressions meaning there is some time left / enough time / a bit of time.

So ako ostane vremena means something like:

• if there is time left
• literally, if some time remains

Here vremena is the genitive singular of vrijeme.

This is very similar to other common patterns:

• Imam vremena. = I have time.
• Nemam vremena. = I don’t have time.

Using vrijeme here would not sound natural in this meaning.

After ako (if), Croatian usually uses the present tense, even when the meaning is future.

So:

• Ako ostane vremena, ... = If there is time / if time remains, ...

This is normal Croatian structure. English often uses a present form too in if-clauses:

• If there is time, we’ll...

So Croatian is actually behaving quite similarly here. The future is shown in the main clause:

• napravit ćemo = we will make

This is about aspect, which is very important in Croatian.

• ostati → perfective
• ostajati / ostaje → imperfective

In Ako ostane vremena, the speaker is talking about a single result: if some time ends up being left over. That is why the perfective form ostane is natural.

ostaje would sound more like an ongoing or habitual process, which does not fit as well here.

So:

• ako ostane vremena = if time ends up remaining / if there is time left
• ako ostaje vremena would sound unusual in this sentence

This is the standard way of writing the future in Croatian with many verbs ending in -ti.

The infinitive is napraviti (to make), but before the clitic future form ćemo, Croatian standard spelling usually drops the final -i:

• napravit ćemo
• radit ćemo
• vidjet ćemo

So:

• napravit ćemo = we will make

You may sometimes see napraviti ćemo, especially from learners or in less careful writing, but standard Croatian prefers napravit ćemo.

Ćemo is the 1st person plural form of the auxiliary verb used to make the future tense.

It means we will.

So:

• napravit ćemo = we will make
• ići ćemo = we will go
• vidjet ćemo = we will see

The full set is:

• ću = I will
• ćeš = you will
• će = he/she/it will
• ćemo = we will
• ćete = you will
• će = they will

Because the preposition od normally takes the genitive case.

• plastelin = plasticine / modeling clay
• od plastelina = out of plasticine

This is the normal way to say what something is made from:

• stol od drveta = a table made of wood
• kuća od kamena = a house made of stone
• vlak od plastelina = a train made of plasticine

Because kuću is the direct object of napravit ćemo, so it must be in the accusative case.

Base form:

• mala kuća = a small house (nominative)

In the sentence, it becomes:

• malu kuću = accusative singular

The adjective must agree with the noun:

• mala → malu
• kuća → kuću

So:

• napravit ćemo malu kuću = we will make a small house

Because vlak is a masculine inanimate noun, and in Croatian those often have the same form in the nominative and accusative singular.

So:

• nominative: vlak
• accusative: vlak

But kuća is feminine, so its accusative singular changes:

• nominative: kuća
• accusative: kuću

That is why you get:

• napravit ćemo vlak
• napravit ćemo malu kuću

Only kuću.

So the sentence means:

• a train made of plasticine and a small house

not:

• a small train and a small house

If you wanted small to describe both nouns, you would need to say it separately for both, for example:

• mali vlak od plastelina i malu kuću

Because Ako ostane vremena is a subordinate conditional clause placed before the main clause.

Croatian normally separates that kind of clause with a comma:

• Ako ostane vremena, napravit ćemo...

This is similar to English:

• If there is time, we’ll make...

If the order is reversed, the comma is often not needed:

• Napravit ćemo vlak od plastelina i malu kuću ako ostane vremena.

Yes. That is also very common and natural.

Both mean roughly if there is time, but there is a slight nuance:

• Ako bude vremena = if there is time
• Ako ostane vremena = if some time is left over / remains

So ostane can suggest that time remains after doing other things first.

Both are correct, but the given sentence slightly emphasizes leftover time.

Because Croatian often leaves out subject pronouns when they are already clear from the verb form.

• napravit ćemo already tells you the subject is we

So adding mi is possible, but not necessary:

• Napravit ćemo... = We will make...
• Mi ćemo napraviti... = We will make... with extra emphasis on we

This is very normal in Croatian.