Čim sjednemo u auto, mama kaže da svi moramo vezati pojas.

Čim means as soon as or once.

It introduces something that happens immediately after something else:

• Čim sjednemo u auto... = As soon as we get in the car...

It is very common in Croatian for actions that follow right away.

Because sjednemo comes from sjesti, which means to sit down / to get seated — a completed action.

• sjesti → sjednemo = we sit down / we get in and sit
• sjediti → sjedimo = we are sitting

So here the idea is not while we are sitting in the car, but the moment we get into the car and sit down.

That is why sjednemo is the natural choice after čim.

Formally, sjednemo is a present-tense form, but here it has future meaning.

This is very common in Croatian after words like:

• čim = as soon as
• kad = when
• ako = if

With a perfective verb like sjesti, the present form often refers to a future completed action:

• Čim sjednemo u auto... = As soon as we get into the car...

So it is present in form, but future in sense.

Because Croatian changes case depending on whether there is:

• movement toward/into somewhere → accusative
• location in somewhere → locative

Here, u auto means into the car, so it uses the accusative:

• u auto = into the car

Compare:

• Sjednemo u auto. = We get into the car.
• Sjedimo u autu. = We are sitting in the car.

So:

• u auto = movement
• u autu = location

Because Croatian does not have articles like English a/an/the.

So Croatian simply says:

• u auto = in/into the car
• pojas = the belt / a belt

The exact meaning depends on context.

This is one of the biggest differences from English: Croatian nouns normally appear without any article.

Because the sentence describes something that happens regularly or typically.

• mama kaže = mom says

In English, we might also say:

• Mom says that...
• Mom always says that...
• Mom tells us that...

Croatian often uses the simple present for repeated or habitual situations. The sentence gives the feeling of a routine: every time they get in the car, mom says this.

Here da means that and introduces a content clause.

• mama kaže da... = mom says that...

So the sentence structure is:

• mama kaže = mom says
• da svi moramo vezati pojas = that we all have to fasten a seat belt

In everyday English, that is often omitted, but in Croatian da is very commonly kept.

Svi means all (of us) or everyone.

In this sentence:

• svi moramo = we all have to

The verb moramo already means we must / we have to, but svi adds the idea that this applies to everyone in the car, not just one person.

So:

• moramo = we have to
• svi moramo = we all have to

Because morati means must / have to, and it is followed by an infinitive.

So the pattern is:

• moramo = we have to
• vezati = to fasten / to tie

Together:

• moramo vezati pojas = we have to fasten the seat belt

This is similar to English:

• we have to go
• we must wait
• we have to fasten...

Croatian often uses this two-part structure with modal verbs.

Yes. Both are possible, but they focus slightly differently.

• vezati pojas = fasten the belt
• vezati se = fasten oneself / buckle up

In everyday speech, people often say:

• Veži pojas.
• Veži se.

Both are natural in the context of a seat belt.

In your sentence, vezati pojas names the object directly, which is very clear and straightforward.

Because each person fastens a seat belt, so Croatian often uses the singular in this kind of statement.

• svi moramo vezati pojas = we all have to fasten a seat belt

This is similar to English, where singular can also be used generically.

Also, pojas here means seat belt from context. Croatian can also say the fuller form:

• sigurnosni pojas = seat belt

But in a car context, just pojas is very natural.

It is in the accusative singular, because it is the direct object of vezati.

We are fastening what?
→ pojas

For this masculine inanimate noun, the accusative singular looks the same as the nominative singular:

• pojas = nominative
• pojas = accusative

So the form does not change, even though the case function does.

Yes. Croatian word order is fairly flexible, though some versions sound more natural or put emphasis in different places.

Your sentence is very natural:

• Čim sjednemo u auto, mama kaže da svi moramo vezati pojas.

Other possible variants might be:

• Čim sjednemo u auto, mama kaže da moramo svi vezati pojas.
• Mama kaže da čim sjednemo u auto svi moramo vezati pojas.

These versions are understandable, but the original sounds smooth and neutral.

Croatian often changes word order for emphasis rather than basic meaning.