Moram oprati zube prije spavanja.

Because Croatian often leaves out subject pronouns when the verb already shows who the subject is.

• Moram already means I must / I have to
• So ja is not necessary

You could say Ja moram oprati zube prije spavanja, but that adds emphasis, something like I have to brush my teeth.

Moram is the 1st person singular present form of morati, meaning must or have to.

It is a very common way to express obligation:

• Moram ići = I have to go
• Moramo učiti = We have to study

In this sentence, it gives the idea of something necessary or obligatory before bed.

After modal verbs like morati, Croatian normally uses the infinitive.

So the structure is:

• moram
  • oprati
• I have to
  • wash/brush

This is similar to English have to brush, except Croatian does not need a separate word like to in the same way English does.

This is a question of aspect, which is very important in Croatian.

• prati = imperfective, focusing on the process or repeated action
• oprati = perfective, focusing on completing the action

Here, oprati zube means brushing the teeth as a completed task. That is why it sounds natural with moram in this context: I need to get my teeth brushed/cleaned.

If you used prati, it would sound less natural here unless you were emphasizing the activity itself or talking about repeated/habitual action.

Yes. Literally, oprati zube is closer to wash/clean the teeth, but in normal Croatian it is a standard way to say brush one’s teeth.

Another possible expression is:

• četkati zube = to brush teeth

The difference is roughly this:

• oprati zube = common everyday way to say you clean/brush your teeth
• četkati zube = emphasizes the brushing action or the toothbrush a bit more

So the sentence sounds completely natural.

Because in Croatian, just like in English, you normally talk about teeth in the plural when brushing them.

• zub = tooth
• zube = teeth, in the form needed here

Since the action is done to the teeth as a direct object, Croatian uses zube in this sentence.

Croatian often leaves possession unstated when it is obvious from context, especially with body parts.

So:

• Moram oprati zube naturally means I have to brush my teeth

Croatian does not need to say my unless you want extra emphasis or contrast.

You could say:

• Moram oprati svoje zube

But that is usually unnecessary and can sound overly explicit unless you are contrasting my teeth with someone else’s.

Spavanja is a noun form, not a verb form.

• spavati = to sleep
• spavanje = sleeping, sleep
• spavanja = the genitive form of spavanje

After prije meaning before, Croatian normally uses the genitive. So:

• prije spavanja = before sleeping / before bedtime

This is a very common Croatian structure:

• prije jela = before eating / before the meal
• prije polaska = before departure

Yes. Croatian has more than one natural way to express this idea.

For example:

• prije spavanja = before sleeping / before bed
• prije nego što zaspim = before I fall asleep
• prije nego što idem spavati = before I go to sleep

These are all possible, but they are not exactly identical in tone:

• prije spavanja is short, neutral, and very common
• prije nego što zaspim is more explicit and clause-based
• prije nego što idem spavati sounds a little more conversational and descriptive

In your sentence, prije spavanja is the most compact and natural version.

Yes. Croatian word order is fairly flexible, though some orders sound more neutral than others.

The most neutral version is:

• Moram oprati zube prije spavanja.

But you could also say:

• Prije spavanja moram oprati zube.
• Zube moram oprati prije spavanja.

These versions shift emphasis:

• Prije spavanja... emphasizes the time
• Zube moram oprati... emphasizes teeth

So the original sentence is a natural, neutral word order.

It is neutral, standard, everyday Croatian.

It would sound normal in speech and writing. It is not especially formal, but it is perfectly correct standard Croatian.

A more casual spoken version might sometimes drop part of the phrase depending on context, but Moram oprati zube prije spavanja is fully natural and idiomatic.