Dok traje terapija, ne smiješ zaboraviti ni jutarnju ni večernju dozu.

Here dok means while or as long as: Dok traje terapija = while the treatment is ongoing.

To say until, Croatian usually uses dok ne + a verb, for example: Čekaj dok ne dođe = Wait until he/she comes.

Both are correct. Croatian word order is more flexible than English word order.

• Dok traje terapija
• Dok terapija traje

Both mean the same thing. The version with traje before terapija sounds very natural in this kind of sentence.

Croatian often leaves out subject pronouns because the verb ending already shows who the subject is.

Here, smiješ means you are allowed / you may in the 2nd person singular, so ti is not needed.

You could say ti ne smiješ, but that would add emphasis, like you must not.

Literally, it means you are not allowed to forget.

The verb smjeti means to be allowed to / may. In instructions, warnings, and medical advice, ne smiješ + infinitive often sounds like:

• you must not
• you shouldn't
• don't

So ne smiješ zaboraviti is stronger than a simple statement. It sounds like an instruction or warning.

Ni ... ni ... means neither ... nor.

So:

• ni jutarnju ni večernju dozu = neither the morning dose nor the evening dose

In Croatian, this usually goes together with a negative verb:

• ne smiješ zaboraviti ni ... ni ...

This is normal Croatian grammar. It is not considered a wrong double negative.

Because Croatian can omit a repeated noun when it is understood from context.

The full version would be:

• ne smiješ zaboraviti ni jutarnju dozu ni večernju dozu

But Croatian often shortens this to:

• ne smiješ zaboraviti ni jutarnju ni večernju dozu

That is completely natural.

Because they agree with dozu, which is feminine singular accusative.

Base forms:

• jutarnja doza = morning dose
• večernja doza = evening dose

But after zaboraviti, the direct object is in the accusative:

• jutarnju dozu
• večernju dozu

So the adjective changes to match the noun.

Because dozu is the accusative singular form of doza.

• doza = nominative singular
• dozu = accusative singular

Since doza is the direct object of zaboraviti, Croatian uses the accusative:

• zaboraviti dozu = to forget the dose

The dictionary form is smjeti.

Its present-tense forms include:

• smijem = I may / I am allowed
• smiješ = you may / you are allowed
• smije = he/she/it may

So ne smiješ means you may not / you must not.

Because zaboraviti is perfective, and that is the natural choice here.

• zaboraviti = to forget (as a complete event)
• zaboravljati = to be forgetting / to forget repeatedly or habitually

In this sentence, the idea is do not let the act of forgetting happen, so zaboraviti fits better.

It is singular informal.

So this sentence is addressed to one person in an informal way.

The formal or plural version would be:

• Dok traje terapija, ne smijete zaboraviti ni jutarnju ni večernju dozu.

Yes. That is also correct.

• Nemoj zaboraviti ... = a direct negative command: Don't forget ...
• Ne smiješ zaboraviti ... = stronger, more like You must not forget ... or You are not allowed to forget ...

So both work, but the tone is slightly different.

Because Croatian has no articles.

So Croatian does not have separate words for a/an or the. Whether something is definite or indefinite is understood from context.

That means:

• terapija can mean therapy or the treatment
• dozu can mean a dose or the dose

The exact meaning depends on the situation.