Questions & Answers about Hann setur dökka úlpu á sig áður en hann fer út.
Both literally mean “on him”, but they’re used differently.
á sig is reflexive: it refers back to the subject of the same clause.
- Hann setur dökka úlpu á sig.
→ He puts a dark coat on himself. (the subject hann is doing something to himself)
- Hann setur dökka úlpu á sig.
á hann would usually refer to another male person, not the subject:
- Hann setur dökka úlpu á hann.
→ He puts a dark coat on him (some other guy).
- Hann setur dökka úlpu á hann.
So here, because he is putting the coat on himself, Icelandic uses the reflexive pronoun sig, not the regular pronoun hann.