Nemoj ući u sobu dok djeca uče.

Nemoj is used to make a negative command to one person in an informal way.

So Nemoj ući... means something like:

• Don’t enter...
• Don’t go into...

In Croatian, a very common way to make a negative imperative is:

• nemoj + infinitive for you singular
• nemojte + infinitive for you plural/formal

Examples:

• Nemoj čekati. — Don’t wait.
• Nemojte govoriti. — Don’t speak.

So in your sentence, Nemoj ući u sobu... is the natural way to say Don’t enter the room...

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

• ući = perfective
• ulaziti = imperfective

Here:

• ući focuses on the completed action of entering
• ulaziti would focus more on the process/repeated action of going in

With a warning like Don’t enter the room, Croatian usually uses the perfective verb ući, because the speaker is stopping the action from happening at all.

So:

• Nemoj ući u sobu. = Don’t go into the room / Don’t enter the room.

The aspect pair is:

• ulaziti — to be going in / to go in repeatedly
• ući — to enter, to go in once and complete it

Because in modern Croatian, the normal and most natural way to make a negative command is:

• nemoj + infinitive

So instead of trying to negate an imperative form directly, Croatian usually says:

• Nemoj ući rather than Ne uđi

For learners, it is safest to remember:

• positive command: Uđi! — Come in! / Enter!
• negative command: Nemoj ući! — Don’t come in! / Don’t enter!

That pattern is extremely common.

Because u can take different cases depending on whether there is:

• movement toward/into something, or
• location inside something

Here we have movement: enter into the room, so Croatian uses the accusative:

• u sobu = into the room

Compare:

• u sobu — into the room, to the room interior
• u sobi — in the room

Examples:

• Ulazim u sobu. — I am going into the room.
• Ja sam u sobi. — I am in the room.

So in your sentence, u sobu is correct because entering involves motion into the room.

Sobu is the accusative singular of soba.

Basic form:

• soba = room

In this sentence:

• u sobu = into the room

A quick mini-paradigm:

• soba — nominative
• sobe — genitive
• sobi — dative/locative
• sobu — accusative

English speakers often notice this because English does not change room like this, but Croatian does.

Here dok means while.

So:

• dok djeca uče = while the children are studying

In many contexts, dok can also relate to until, depending on the sentence, but in this example the meaning is clearly while/during the time that.

So the structure is:

• Nemoj ući u sobu dok djeca uče.
• Don’t enter the room while the children are studying.

Because uče is already a conjugated present-tense verb meaning are studying / study.

So:

• djeca = children
• uče = study / are studying

Croatian does not need a separate verb like English are in the present continuous.

English:

• The children are studying.

Croatian:

• Djeca uče.

That is a complete clause by itself.

Because djeca means children, and it takes plural agreement.

So we say:

• Djeca uče. — The children are studying.

not:

• Djeca uči.

This word is a little unusual for learners because its form does not look like a typical plural noun, but grammatically it behaves as plural.

Very useful comparison:

• dijete = child
• djeca = children

Uče is the 3rd person plural present tense of učiti.

So the forms are:

• učim — I study
• učiš — you study
• uči — he/she studies
• učimo — we study
• učite — you plural/formal study
• uče — they study

Since djeca means children, the sentence uses the they form:

• djeca uče — the children study / are studying

It can mean both, depending on context.

Croatian present tense often covers both:

• they study
• they are studying

In this sentence, because of the situation and dok (while), the natural English meaning is:

• while the children are studying

So Croatian does not need a special continuous form like English does.

Because Croatian has no articles like English a/an/the.

So:

• soba can mean a room or the room
• djeca can mean children or the children

The exact meaning depends on context.

In your sentence, English naturally uses the:

• the room
• the children

But Croatian does not need separate words for that.

Yes, that is possible, but it is a bit different in form and feel.

• Nemoj ući u sobu... = Don’t enter the room.
• Ne ulazi u sobu... = Don’t go into the room / Don’t be going into the room.

The first one, with nemoj + infinitive, is the very standard negative command pattern learners should know well.

The second one uses an imperative-style negative with the imperfective verb and can sound more like a warning against the action/process or repeated action.

For most learners, Nemoj ući u sobu is the safest and most straightforward version.

Croatian word order is fairly flexible, but the original order is natural and clear:

• Nemoj ući u sobu dok djeca uče.

You may sometimes hear variations for emphasis, such as:

• Dok djeca uče, nemoj ući u sobu.

This puts more emphasis on the time clause while the children are studying.

But for neutral everyday usage, the original sentence is perfectly natural.

Nemoj is for one person, in an informal way.

If you were speaking to:

• one person informally: Nemoj ući...
• several people or one person formally: Nemojte ući...

So:

• Nemoj ući u sobu. — Don’t enter the room. (to one person, informal)
• Nemojte ući u sobu. — Don’t enter the room. (to several people, or one person formally)

That distinction is very important in Croatian.