The latter is Hon vet vad hon vill ha, since
- vill = want
- vill ha = want to have
And, unlike in English, "want" cannot cover both.
"Sin" is a possessive pronoun meaning her/his/its/their reflecting back on the subject.
"Hon" is a personal pronoun meaning she
In this case there is no possessive pronoun in the sentence. (She knows what she wants)
I see that "vad" has the same form both as a relative and interrogative pronoun. Does "när" also stay the same?
I have the impression of having seen somewhere in the internet, that when one use e.g. ''vad'' or ''vem'' as relative pronoun, one should put a ''som'' right after it. So, according to what I read before, it should be: ''Hon vet vad som hon vill.''. Does that even make sense?
Sounds like an error to me, but it does work for vem: Jag vet vem som har gjort det 'I know who did it'.
But is that a must to use ''som'' with ''vem'', or I could just say: ''Jag vet vem gjort det.'' ?
You can't start a relative clause with vem. We use som for that:
'The man who is sitting there is my dad' - Mannen som sitter där är min pappa
You also can't add vem in the latter sentence.
The reason Jag vet vem som har gjort det works is that in this sentence, vem is not a relative pronoun, i.e. it does not refer to something that has been said in the clause previously. In Mannen som sitter…, som points back to mannen, who was mentioned. But for vem in Jag vet vem…, there is nothing like that to point back to.