Learn Hardcore Swedish: Jag har en kollega vars mobil ofta är avstängd. - I have a colleague whose mobile is often turned off.

It’s invariable (same form for all genders and numbers).
It can refer to people or things: en kollega vars mobil …, ett företag vars huvudkontor …
You don’t use som together with vars; vars already does the linking.
Don’t confuse vars (relative “whose”) with vems (interrogative “whose?” used in questions).

Why is it vars mobil and not vars mobilen?

After any possessive (including vars, min, din, hans, hennes, sin), Swedish does not use an article or the definite ending on the noun. So you say:

vars mobil, min bil, hans bok (no -en/-et on the noun) If there’s an adjective, it takes the definite ending -a, but the noun still stays “indefinite” in form:
vars nya mobil, min röda bil, hennes gamla telefon

Why is the word order mobil ofta är and not mobil är ofta?

Because this is a subordinate (relative) clause. In Swedish subordinate clauses, sentence adverbs like inte, ofta, alltid, kanske come before the finite verb:

Relative clause: … vars mobil ofta är avstängd. Compare a main clause, where the verb is in second position and the adverb comes after it:
Main clause: Mobilen är ofta avstängd.

Can I rewrite it with som instead of vars?

Yes. Two natural rewrites:

Jag har en kollega som ofta har sin mobil avstängd. (use reflexive sin to show it’s the colleague’s own phone)
Jag har en kollega som ofta har mobilen avstängd. (also common; context usually makes it clear it’s their phone) The vars-version makes the phone the subject of the relative clause; the som-version makes the colleague the subject. Both are idiomatic.

Should I write som vars?

No. Vars itself is the relative linker for possession. You don’t combine it with som.

Does avstängd agree with mobil? What about plural or neuter? Do predicatives take the definite -a?

Predicative adjectives/participles agree in gender/number:
- Common gender singular: avstängd (en mobil är avstängd)
- Neuter singular: avstängt (ett larm är avstängt)
- Plural: avstängda (mobilerna är avstängda)
Predicatives do NOT take the definite -a just because the noun is definite: Mobilen är avstängd (not “avstängda”).

Is avstängd the only or best way to say “off” for a phone?

For phones and many devices, avstängd is the most standard “switched off.”

av can mean “off” and is common in speech, but avstängd sounds clearer/more standard for a phone.
avslagen (from slå av) exists but is less common for phones; släckt is for lights/candles (not phones).
Opposites: på, påslagen, igång (“on/turned on/running”).

Can vars refer to plural or inanimate antecedents?

Yes. Vars is fine with plural and inanimate antecedents:

Jag har kollegor vars mobiler ofta är avstängda.
Ett företag vars produkter säljer bra. In formal writing you may also see plural vilkas, but vars is standard for both singular and plural.

Do I need a comma before the relative clause?

No comma for a restrictive relative clause in modern Swedish:

Jag har en kollega vars mobil … (no comma) Commas can appear with clearly non-restrictive clauses, but that’s rarer and more stylistic; here it’s restrictive.

Is vars formal? What about vilkens/vilkets/vilkas?

Vars is neutral and by far the most common genitive relative pronoun. The forms vilkens/vilkets/vilkas exist (especially vilkas for plural) but sound formal or old-fashioned; most of the time, use vars.

What gender is kollega, and how do I inflect it?

Gender: common (en-ord) → en kollega
Definite singular: kollegan
Indefinite plural: kollegor
Definite plural: kollegorna
Genitive: add -s → kollegas, kollegornas

Could I express this with an s-genitive instead?

Yes:

Min kollegas mobil är ofta avstängd. This is a bit more direct if you mean a specific colleague you’re already “anchoring” as yours. Jag har en kollega vars … is a natural way to introduce a new, unspecified colleague in discourse.

What’s the difference between är avstängd, stängs av, and blir avstängd?

är avstängd = state: “is switched off.”
stängs av = repeated or passive action: “is (being) turned off” (habitually or at some moment).
blir avstängd = change of state: “gets/becomes switched off.”

Where would inte go if I want to say “whose phone is not switched off”?

In a subordinate clause, put inte before the finite verb:

… vars mobil inte är avstängd. Compare a main clause:
Mobilen är inte avstängd.

Any pronunciation tips?

vars: the rs is usually one retroflex sound, like “varsh.”
kollega: stress on the second syllable: ko-LE-ga.
mobil: stress on the second syllable: mo-BIL.
avstängd: the ä like the vowel in English “men”; final -ngd is pronounced as in Swedish trängd.

What’s the difference between ofta and oftast?

ofta = “often” (frequent).
oftast = “most often/usually” (the most common case). You could say … vars mobil oftast är avstängd if you mean “most of the time.”