Dok traje terapija, odmaraj se što više i ne vraćaj se na posao dok ti ne bude bolje.

Not quite.

• In Dok traje terapija, dok means while / as long as.
• In dok ti ne bude bolje, dok ... ne means until.

So the sentence uses the same conjunction in two common time meanings:

• dok + present → while
• dok + ne + verb → until

That second pattern is very common in Croatian:

• Čekaj dok ne dođem. = Wait until I come.
• Ne idi van dok ne prestane kiša. = Don’t go out until the rain stops.

Croatian word order is much freer than English word order.

Both are possible:

• dok traje terapija
• dok terapija traje

They mean the same thing. After a conjunction like dok, it is very normal for the verb to come before the subject. So traje terapija sounds natural and standard.

Literally, it is while lasts the therapy, but in natural English we say while the therapy lasts or while you are undergoing treatment.

Because terapija is the subject of traje.

The verb trajati means to last. The thing that lasts is the subject, so it stays in the nominative:

• terapija traje = the therapy lasts

So in dok traje terapija, terapija is nominative singular.

This is mainly about aspect.

• odmarati se = imperfective
• odmoriti se = perfective

Here the speaker is giving advice for an ongoing period: while the therapy lasts, keep resting / rest regularly / rest as much as possible. That is why the imperfective imperative odmaraj se is natural.

Compare:

• Odmori se! = Have a rest! / Rest up!
  More like one complete act.
• Odmaraj se. = Rest. / Keep resting.
  More ongoing, repeated, or habitual.

Because the sentence talks about a period of recovery, odmaraj se fits better.

Usually not literally.

In Croatian, many verbs are used with se as part of the verb itself. In these cases, English often does not use self at all.

Here:

• odmarati se = to rest
• vraćati se = to return / go back

So se is part of the normal form of these verbs. You generally should learn them together:

• odmarati se
• vratiti se / vraćati se

Sometimes se is truly reflexive, but often it is just how the verb works in Croatian.

što više here means as much as possible or as much as you can.

So:

• odmaraj se što više = rest as much as possible

In this pattern, što is not the question word what. It acts as an intensifier in comparisons and set phrases.

You will also see:

• što prije = as soon as possible
• što brže = as quickly as possible
• što manje = as little as possible

Again, this is mostly about aspect and the type of prohibition.

• vraćati se = imperfective
• vratiti se = perfective

Ne vraćaj se na posao sounds like don’t go back to work during this period / don’t be returning to work yet. It gives an ongoing prohibition, which fits the context.

Also, in modern Croatian, when a negative command involves a perfective verb, speakers often prefer:

• nemoj se vratiti rather than ne vrati se

So these are natural options:

• ne vraćaj se na posao dok ti ne bude bolje
• nemoj se vratiti na posao dok ti ne bude bolje

Both are good, but the sentence you were given uses the imperfective negative imperative.

Because Croatian uses vratiti se / vraćati se na posao for to return to work.

With motion toward something, Croatian often uses:

• na + accusative

So here:

• na posao = to work / back to work

Compare:

• Idem na posao. = I’m going to work.
• Vraćam se na posao. = I’m returning to work.
• Na poslu sam. = I’m at work.

Notice the difference:

• na posao = motion toward work
• na poslu = location at work

Ti is in the dative case, and it marks the person experiencing the state.

Croatian very often says things like:

• Dobro mi je. = I feel well. / I’m fine.
• Bolje mi je. = I feel better.
• Bit će mi bolje. = I’ll feel better.

Literally, bolje ti je is something like it is better to you, but natural English is you feel better.

So:

• dok ti ne bude bolje = until you feel better

This dative construction is very common for physical and emotional states.

Because Croatian commonly uses bude in future-oriented subordinate clauses like this.

After conjunctions such as kad, dok, čim, and ako, Croatian often uses forms that point to a future result or change:

• Dođi kad budeš mogao. = Come when you can.
• Čekaj dok ne bude gotovo. = Wait until it is finished.
• Ne vraćaj se dok ti ne bude bolje. = Don’t come back until you feel better.

Here, bude refers to a future state: at some later point, your condition will improve.

Why not nije bolje?

• nije describes a present state: it is not better
• ne bude points to a future point: until it becomes/is better

And budeš bolje is not the usual way to say this. Croatian normally uses the impersonal expression bude ti bolje, not budeš bolje.

Because the idea is better, not necessarily fully well.

• dobro = well / fine
• bolje = better

If someone is recovering, Croatian very naturally says:

• Bolje mi je. = I feel better.

That does not mean the person is completely healthy yet. It just means there has been improvement.

So:

• dok ti ne bude bolje = until you feel better / until you get better

If you said dok ti ne bude dobro, that would mean something closer to until you feel well/fine, which is a bit stronger.

Because Dok traje terapija is a subordinate time clause placed before the main clause.

Croatian normally separates that kind of introductory clause with a comma:

• Dok traje terapija, odmaraj se...

That comma is very natural and expected.

You may also notice there is no comma before the second dok:

• ...i ne vraćaj se na posao dok ti ne bude bolje.

That final clause is closely tied to ne vraćaj se na posao, so it is commonly written without a comma.

Yes. They are 2nd person singular imperatives, so the speaker is talking to one person:

• odmaraj (ti) = rest
• vraćaj se (ti) = return / go back

Croatian usually leaves the subject pronoun out, so there is no written ti before the imperatives. It is understood from the verb form.

If the speaker were addressing more than one person, you would expect plural forms such as:

• odmarajte se
• ne vraćajte se