Learn Hardcore Croatian: Ne smiješ zatvoriti vrata dok djeca dolaze. - You may not close the door while the children are coming.

It’s the 2nd person singular present of smjeti (to be allowed/permitted). Ne smiješ means “you’re not allowed to / you must not,” i.e., a prohibition based on rules or permission. Positive: Smiješ zatvoriti vrata = “You’re allowed to close the door.”

How is ne smiješ different from ne možeš?

Ne smiješ = you’re not allowed (prohibition).
Ne možeš = you can’t/are unable (lack of ability or possibility). Example: Ne smiješ zatvoriti vrata (rule forbids it) vs Ne možeš zatvoriti vrata (you’re unable, e.g., they’re jammed).

Where does the negation go with a modal — can I say Smiješ ne zatvoriti vrata?

Normally the negation goes on the modal: Ne smiješ zatvoriti vrata = you must not close the door.
If you say Smiješ ne zatvoriti vrata, it means “you’re allowed not to close the door” (permission to refrain), which is a different meaning.

Why is the perfective zatvoriti used here instead of the imperfective zatvarati?

Zatvoriti (perfective) focuses on the completed result: don’t make the door end up closed.
Zatvarati (imperfective) focuses on the ongoing action: don’t be closing the door while they’re coming.
Both are possible:
Ne smiješ zatvoriti vrata dok djeca dolaze. (don’t complete the closing)
Ne smiješ zatvarati vrata dok djeca dolaze. (don’t engage in the closing action during that time)
With dok “while,” many speakers prefer the imperfective for the sense of ongoing overlap, but your sentence with the perfective is also fine.

Is there a more direct way to say “Don’t close the door …”?

Yes, use the imperative:

Ne zatvaraj vrata dok djeca dolaze/ulaze.
Nemoj zatvarati vrata dok djeca dolaze/ulaze.
These sound like a direct instruction rather than a rule about permission.

Why is vrata plural? Does it mean multiple doors?

Vrata is a plural-only noun (pluralia tantum) in Croatian and typically means “the door” (as a unit). There are no articles in Croatian, so vrata can correspond to English “the door” or “the doors,” depending on context.

What case is vrata in here, and why doesn’t it change form?

After a transitive verb like zatvoriti, the object is in the accusative. For neuter plural vrata, the nominative and accusative look the same: vrata. Other cases do change: s vratima (with the door), o vratima (about the door), prema vratima (toward the door).

Why does the verb agree in the plural: djeca dolaze?

Djeca (children) is grammatically plural, so verbs and pronouns agree in the plural: Djeca dolaze (The children are coming). Adjectives with it are neuter plural: dobra djeca (good children).

What if it’s one child?

Use dijete (neuter singular): Ne smiješ zatvoriti vrata dok dijete dolazi.
For “until the child arrives,” you’d say: … dok dijete ne dođe.

Why is it dok djeca dolaze and not dok djeca dođu?

Dok djeca dolaze uses the imperfective present and means “while the children are coming” (overlapping in time).
Dok djeca dođu by itself is not how you express “until” in Croatian. For “until,” you normally use dok ne
- perfective: see next answer.

How do I say “until the children arrive”?

Use dok ne + perfective: Ne smiješ zatvoriti vrata dok djeca ne dođu.
Note: the ne belongs to the conjunction dok ne and does not make the whole sentence “double negative”; it simply signals “until.”

Do I need a comma before dok?

Temporal “while”: no comma when the dok-clause follows the main clause: Ne smiješ zatvoriti vrata dok djeca dolaze.
If the dok-clause comes first, put a comma after it: Dok djeca dolaze, ne smiješ zatvoriti vrata.
Contrastive “whereas” meaning takes a comma: On se igra, dok ja radim.

Can I move parts of the sentence around?

Yes, word order is flexible for emphasis:

Dok djeca dolaze, ne smiješ zatvoriti vrata. (fronts the time frame)
Ne smiješ zatvoriti vrata dok dolaze djeca. (end-focus on djeca) The original order is the most neutral.

Would ulaze be more natural than dolaze here?

Often, yes. With doors, ulaziti (“to enter”) is very idiomatic:

Ne smiješ zatvoriti vrata dok djeca ulaze.
Dolaze (“are coming/approaching”) is still correct; it just focuses on their approach rather than the act of entering.

Can I replace vrata with a pronoun?

Yes. Use the clitic ih (accusative plural):

Ne smiješ ih zatvoriti dok djeca dolaze/ulaze.
Clitics like ih go after the first stressed element in the clause (here, after smiješ).

How do I refer to a future situation with dok?

For future time, Croatian still commonly uses the present in the dok-clause:

Ne smiješ zatvoriti vrata dok djeca ulaze sutra.
A more formal/explicit future is also possible:
Ne smiješ zatvoriti vrata dok djeca budu ulazila. (literally “while the children will be entering”)