Learn Hardcore Norwegian: Barna gråter når filmen slutter.

It means “the children.” The noun barn (child) is neuter and has no ending in the plural (indefinite plural = barn). To make it definite plural (“the children”), you usually add -a: barna. So:

et barn = a child
barnet = the child
barn = children
barna = the children

Is “barnene” also correct, or must it be “barna”?

In Bokmål, both barna and barnene are accepted as the definite plural. Barna is far more common and usually sounds more natural. In Nynorsk, only barna is correct.

What’s the singular of “barna,” and how do all the forms of “barn” look?

Singular indefinite: et barn (a child)
Singular definite: barnet (the child)
Plural indefinite: barn (children)
Plural definite: barna / barnene (the children)

Why is it “filmen” with -en? What gender is “film,” and what are its forms?

Film is a masculine noun in Bokmål. Its common forms are:

en film (a film)
filmen (the film)
filmer (films)
filmene (the films) “Filma” would be a feminine definite form, but “film” is not normally used as feminine in Bokmål.

Why is it “når” here and not “da”? What’s the difference?

når = when (general time, present/future, or repeated/habitual events)
da = when (single, specific event in the past) Your sentence describes a general/whenever situation, so når fits. For a single past event, use da: Barna gråt da filmen sluttet (The children cried when the movie ended).

Both verbs are in the present tense—why? Could I use the future?

Norwegian often uses the present to state general truths or habitual actions, and to talk about the future in time clauses. Barna gråter når filmen slutter is natural. You could predict the future with expressions like kommer til å or vil, but you still typically keep present in the time clause:

Barna kommer til å gråte når filmen slutter.

How do I say it in the past: “The children cried when the movie ended”?

Use the past and switch to da:

Barna gråt da filmen sluttet. Colloquial past for “slutte” can also be slutta:
Barna gråt da filmen slutta.

Can “når” also mean “if”? Should I use “om” or “hvis”?

Use hvis or om for “if.” Når means “when.”

If: Barna gråter hvis/om filmen slutter for tidlig.
When: Barna gråter når filmen slutter. (“om” also means “about/whether,” so hvis is often the clearest “if.”)

Is the word order here following the V2 rule? What about inside the “når”-clause?

Main clause (V2): The finite verb is in second position. Barna (subject) gråter (verb) … = correct V2.
Subordinate clause with når: Subject typically comes before the verb. når filmen slutter = conjunction + subject + verb, which is the normal order for a subordinate clause.

Where does “ikke” go if I want to negate the sentence?

Main clause: Barna gråter ikke når filmen slutter. (verb + ikke)
Subordinate clause: Når filmen ikke slutter i tide, gråter barna. In subordinate clauses, ikke comes before the finite verb: conjunction + subject + ikke + verb.

Could I say “er ferdig/er slutt/er over” instead of “slutter”?

Yes, with slight nuances:

er slutt = is over/finished (common and idiomatic: Filmen er slutt)
er ferdig = is finished/done (neutral)
er over = is over (time/event is over) All are fine; slutter emphasizes the event of ending, while the others describe the state after the ending.

What’s the difference between “slutter” and “ender”?

slutte is the everyday verb “to stop/end/finish” for events, activities, schedules: Filmen slutter klokka ni.
ende often appears as ende med (å) “end with/end up (doing)”: Filmen ender trist or Filmen ender med en overraskelse. Both can translate “end,” but slutte is the default for something that comes to an end at a time.

Is there a progressive form like English “are crying”?

Norwegian usually uses the simple present for ongoing actions: Barna gråter = “The children are crying.” To highlight the ongoing aspect, you can use posture + “og”:

Barna sitter/står/ligger og gråter. Avoid using holder på å gråte for a neutral progressive; it tends to mean “are about to cry” or “almost crying.”

How do I pronounce the sentence naturally?

Barna: roughly “BAR-na.” In many dialects, the “rn” merges into a single retroflex sound.
gråter: “GROH-ter.” The å is like a long “oh.”
når: “nohr,” with a long “o/å”-like vowel.
filmen: “FIL-men.”
slutter: “SLUHT-ter,” with the Norwegian u as a fronted “oo” (like German “ü”), and a clear double “t.” Stress is on the first syllable of each word.

Are there more colloquial ways to say “children,” like “kids”?

Yes:

ungene (the kids) is common and informal: Ungene gråter når filmen slutter.
unger is the indefinite plural “kids.” barn/barna is neutral and standard; ungene is friendly/colloquial.

Do I need a comma before “når”?

Not when the subordinate clause comes after the main clause, as in your sentence—no comma is needed. If you front the time clause, you do use a comma:

Når filmen slutter, gråter barna.

Could I make it generic: “Children cry when a movie ends”?

Yes. Use the indefinite plural for “children” and an indefinite for “movie”:

Barn gråter når en film slutter. Your original (with definites) refers to specific children and a specific movie already known in context.

Can I front the “når”-clause for emphasis?

Yes:

Når filmen slutter, gråter barna. Note the V2 rule in the main clause still applies: after fronting the time clause, the finite verb gråter remains in second position, so the subject barna comes after it.

Do I ever add “så” after “når,” like “Når filmen slutter, så gråter barna”?

You can, and it’s common in spoken Norwegian for emphasis or rhythm. In careful writing, it’s usually omitted:

Spoken/colloquial: Når filmen slutter, så gråter barna.
Neutral/standard: Når filmen slutter, gråter barna.