"I find it hard to appreciate his opinions."
Translation:Jag har svårt att uppskatta hans åsikter.
No, sina is used as a reflexive possessive adjective that refers back to something earlier in the sentence. Sina would be used to mean "his/her" when "he/she" is mentioned earlier in the sentence. For example, Han älskar sin fru (He loves his wife). If you said, Han älskar hans fru, that would mean that he loves someone else's wife. It would not even be grammatically correct in the sentence you asked your question about (it could be grammatically correct but nonsensical in other sentences).
No, we don't say that. There is an expression Jag finner det svårt att … but it's very formal.
I think it's the first time I come across this expression. How would that translate? It can't be "I have difficult"?
Maybe compare it to the expression "I am having difficulty to..."? Also my first time seeing this expression, but I thought it made sense anyway.
Well, it's optional. It's idiomatic to use it but it sounds perfectly fine not to.
Why is "Jag har svårt för att..." also right? Is there a difference between using för and leaving it out? Sounds like "I'm having a hard time because...".
Får man säga "jag har det svårt" istället?
Something closer to English I mean. It was marked wrong so I assume it's actually wrong...