Since skjorte has the feminine form skjorta the possessive pronoun is si, it points back to the gender of the object (the shirt). Skjorte can also get the masculine form skjorten (it's a dialectical preference whether to choose m or f) and the possessive would be sin
Sitt applies to neuter nouns.
LOL Can you imagine it?! I'll be the one with a skjorte wrapped around my waist, 'cause I still get that and the word for skirt mixed up! In my mind the feminine shirt is white with outlines of purple flowers. I will forever remember the gender of shirt that way now, so thank you.
Well that's true. There doesn't seem to be any rhyme nor reason to it most of the time. However, you have helped to cement the fact that skjorte is feminine in my brain now, by referring to the 'feminine shirt' lol (Unfortunately, I didn't know any Norwegian when I was in Norway. You should have seen the charades involved in trying to ask where the cemetery was! But I also expect to go back.)
I use rense about cleaning stuff without the use of water/fluids (or with minimal amount of fluids). And sometimes if it is an especially thorough sort of washing. I would normally be satisfied with washing my hair (vaske håret), but every now and then I'd want to cleanse it (rense håret). Yes, I just discovered that we would say rense for both cleanse and clean.
You wash your hands and clean your nails. Du vasker hendene og renser neglene.
A dry-cleaner is et renseri.
I know you're an expert at level 5, but "si" does not translate to "his own".
It is the subjects shirt, yes, but "own" should not be explicitly added in the translation, as it isn't in the Norwegian sentence. That would be "sin egen".
Adding "own" is a good way of explaining the concept to non-natives who aren't used to having two different possessive pronouns for "his", but it's not a natural-sounding translation. Example: "He put on his own shoes and his own jacket and went to see his own friends."