Learn Hardcore Norwegian: Det var hun som satte på vaskemaskinen i morges. - It was she who started the washing machine this morning.

It’s a cleft construction used to add emphasis or to answer a specific “who/what/when” question. Det var hun som … highlights hun as the focus: She (and not someone else) did it. You can cleft other parts too:

Det var i morges hun satte på vaskemaskinen. (emphasizes the time)
Det var vaskemaskinen hun satte på i morges. (emphasizes the object)

Why is it hun (subject form) after var, not henne?

After the copular verb å være (to be), standard Norwegian uses the subject form: Det er/var jeg/du/han/hun/vi/dere/de. So Det var hun … is the recommended form. In casual speech you may hear Det var henne …, but in writing and careful speech, prefer hun.

What is som doing, and why not hvem?

Som is the relative marker linking the focused element to the clause. It means roughly “who/that.” In clefts like this, som is required. Hvem is a question word and doesn’t introduce relative clauses in this way, so Det var hun hvem … is incorrect.

Note: When som functions as the subject of the relative clause (as here), it is obligatory. When som is the object, it can often be dropped in informal style:

Subject: Det var hun som satte på … (must have som)
Object: Det var vaskemaskinen (som) hun satte på …

What’s the word order inside the som-clause?

In a som-clause, you use subordinate clause order: subject before the finite verb. When som itself is the subject, it’s followed by the verb:

Correct: … som satte på vaskemaskinen …
Don’t add another subject pronoun there; not: … som hun satte … (unless som is the object of the clause, which it isn’t in the original sentence).

Where does negation (ikke) go, and what difference in meaning does it make?

Det var ikke hun som satte på vaskemaskinen i morges. = It wasn’t she who did it (someone else did).
Det var hun som ikke satte på vaskemaskinen i morges. = She is the one who didn’t do it.

Inside the som-clause, place ikke early: som ikke satte …

Is sette på like an English phrasal verb? How is it different from slå på and skru på?

Yes, sette på is a particle verb. Typical nuances:

sette på: start a process/appliance or “put on” media: sette på vaskemaskinen, sette på en vask (start a wash), sette på kaffen, sette på musikk.
slå på: switch on power: slå på lyset/TV-en.
skru på: turn on by turning a knob/tap: skru på kranen/radioen.

With a washing machine, sette på (en vask/vaskemaskinen) is very idiomatic for starting a program; slå på can sound like just powering it up.

What’s the difference between å sette and å sitte? The past forms look confusing.

å sette (to set/put/place/start): setter – satte – har satt.
å sitte (to sit): sitter – satt – har sittet.

Notice: satte (past of sette) vs satt (past of sitte, and past participle of sette).

Where does the particle på go with objects and pronouns?

With a noun object, the particle stays right after the verb: Hun satte på vaskemaskinen.
With a pronoun object, the pronoun typically goes between verb and particle: Hun satte den på. Similarly: Hun slo den av.
Using Hun satte på den is generally avoided.

Why is it the definite form vaskemaskinen?

Because a specific, known machine is meant (e.g., the one at home). Forms:

en vaskemaskin (a washing machine)
vaskemaskinen (the washing machine)
vaskemaskiner (washing machines)
vaskemaskinene (the washing machines)

Note: You’ll also hear sette på en vask (“start a wash [load]”), which focuses on the laundry cycle rather than the machine itself.

Does i morges mean “tomorrow”? I get confused with morgen.

i morges = this morning (earlier today). You can say this even in the afternoon/evening of the same day.
i går morges = yesterday morning.
i morgen = tomorrow.
i morgen tidlig = tomorrow morning (early).
Habitual time: om morgenen (“in the mornings”), på morgenen can mean “in the morning” on a particular day.
Many people say i dag tidlig for “earlier this morning” too. I dag morges is common in speech but often discouraged in careful writing as redundant.

Can I move i morges to other positions?

Yes:

I morges var det hun som satte på vaskemaskinen.
Det var i morges hun satte på vaskemaskinen.
Det var hun som satte på vaskemaskinen i morges. All are correct; fronting or clefting changes focus/emphasis.

Can I say Hun var den som satte på vaskemaskinen i morges?

Yes. Hun var den som … means “She was the one who …” It’s close in meaning to Det var hun som …, though det-clefts are extremely common and often feel a bit lighter and more neutral.

Why is it Det var … and not Den var … at the start?

In clefts, det is a dummy pronoun (an expletive) and does not agree in gender with the focused element. You say:

Det er jeg/du/han/hun … som … Using den there would suggest you’re referring back to a specific common-gender noun, which you aren’t.

Do I need a comma before som?

No. Norwegian does not use a comma before som in restrictive relative clauses like this: Det var hun som satte på … (no comma).

Is there any pronunciation tip for vaskemaskinen and i morges?

vaskemaskinen: the sk before i is pronounced like English “sh”: va-ske-ma-shi-nen. Primary stress on the first syllable: VAS-ke-ma-shi-nen.
i morges: commonly pronounced like “ee MOR-ess” (the g is silent).