Learn Hardcore Norwegian: De kjeder seg ikke når de tilbringer tid med venner. - They are not bored when they spend time with friends.

In a main clause, ikke usually comes after the finite verb. With reflexive verbs, the reflexive pronoun typically sits right after the verb, and ikke follows that:

Standard: De kjeder seg ikke.
De kjeder ikke seg sounds off/unidiomatic in standard written Norwegian.

With an auxiliary, ikke comes after the auxiliary:

De har ikke kjedet seg.

What’s the difference between når and da?

når = when/whenever for present, future, and habitual/general time. That’s why når de tilbringer … works here.
da = when (for a single, specific event in the past). Example in the past: De kjedet seg ikke da de tilbrakte tid med venner i går.

Why is de repeated? Could I drop the second de?

No. Norwegian doesn’t allow the English-style “-ing” clause without a subject. The subordinate clause after når needs its own subject:

Correct: De kjeder seg ikke når de tilbringer tid med venner. You can also front the når-clause:
Når de tilbringer tid med venner, kjeder de seg ikke.

What happens to word order if I put the når-clause first? Do I need a comma?

Yes, add a comma, and follow the V2 rule (the finite verb is the second element in the main clause):

Når de tilbringer tid med venner, kjeder de seg ikke. Not standard in writing: Når de tilbringer tid med venner, de kjeder seg ikke.

Could I use mens instead of når?

Often, yes. mens = while (simultaneous actions). It fits when the meaning is “They don’t get bored while they spend time with friends”:

De kjeder seg ikke mens de tilbringer tid med venner. Nuance: når can sound more general/habitual (“whenever”), while mens emphasizes simultaneity.

Why tilbringer tid? Could I say bruker or something like “spend”?

tilbringe (tid) med noen = spend time with someone. That’s the direct equivalent for people.
bruke tid på noe/å + infinitive = spend time on something/doing something.
Everyday alternative: er sammen med or informal henger med. Avoid false friends: spandere means “to treat/pay for,” not “to spend (time).”

Is med venner definite or indefinite? How would I say with their friends?

med venner is indefinite plural (“with friends” in general). To say “with their (own) friends,” use the reflexive possessive:

med vennene sine = with their own friends (the subject’s friends) If you mean someone else’s friends:
med vennene deres = with their friends (another group’s) You can also say med noen venner = with some friends. Adding sammen is fine: (sammen) med venner.

Does Norwegian distinguish between simple and progressive like English? How do I say “aren’t getting bored”?

Norwegian present covers both simple and progressive. De kjeder seg ikke can mean “they don’t get bored” or “they aren’t getting bored,” depending on context. Add time words for clarity:

akkurat nå (right now), for tiden (these days)

Why de and not dem?

de is the subject form; dem is the object form.

Subject: De kjeder seg ikke.
Object: Jeg ser dem. Note: A capitalized De can be a very formal “you” in writing; it’s rarely used today.

How would I say this in the past?

å kjede seg: kjeder seg (present), kjedet seg (past), har kjedet seg (perfect)
å tilbringe: tilbringer (present), tilbrakte (past; also tilbragte is accepted), har tilbrakt (perfect)

Example: De kjedet seg ikke da de tilbrakte tid med venner.

Where does ikke go in a subordinate clause?

In subordinate clauses, ikke comes before the finite verb:

De kjeder seg når de ikke tilbringer tid med venner. Compare the main clause: De kjeder seg ikke … (ikke after the verb).

Can I omit tid after tilbringer?

No. tilbringe is transitive: you need an object like tid, kvelden, dagen, helgen.

Natural: tilbringer tid med venner / tilbringer kvelden med venner
Not natural: tilbringer med venner

What’s the difference between kjeder seg, kjedelig, and lei?

å kjede seg = to be bored (state of the person): Jeg kjeder meg.
kjedelig = boring (describes something/someone): Filmen er kjedelig.
lei = fed up/tired of: Jeg er lei av dette. Be careful: Jeg er kjedelig means “I am boring,” not “I am bored.”

Any quick pronunciation tips for this sentence?

kj- in kjeder is a soft sound (like the German “ich,” not like English “ch”): roughly [ç].
de is typically pronounced like “di.”
ikke sounds like “IK-keh.”
Final -r is usually pronounced in Bokmål-standard speech.

Is this Bokmål or Nynorsk? What would it look like in Nynorsk?

Your sentence is Bokmål. A common Nynorsk version:

Dei kjedar seg ikkje når dei tilbringar tid med vener. Key swaps: Dei (they), kjedar, ikkje, vener (friends).