"How do you feel?". Third person, sign of respect, typical sentence at the doctor, they use a third person with a patient.
Ciao Federico! Yes, as the case at the doctor. But they do not use the third person, but the formal you (Lei, with capital L always) and the conjugation is the same as the third person (in some books, you can see lui, lei, Lei as the same conjugation but they represent the third person for lui and lei, he and she, and the formal second person Lei for both masculine and feminine genres, as formal you, the same as the Spanish usted). So you cannot assume they (doctors) are using the third person but still the formal second person to show respect.
For the second person (formal): (Lei) Come si sente?
And for the third person: (lui or lei) Come si sente?
How can i distinguish the impersonal si (= one, someone) from the reflexive pronoun si (=he, she)? I wrote "how does one feel?" And is marked as error suggesting "how does he feel?" as the correct answer.
In Italian, in this case, since there is ambiguity, you use "uno" to put emphasis on a generic "one". So, that would be "Come si sente uno?". It's kind of a rhetorical question though.
In general, without a broader context, it is impossible to tell.
You could rephrase the English as, "How does he feel about himself?". The "si" is the reflexive bit that makes this question different from "How does he feel/hear something else". I'm not a native speaker, but this is how I understand it:
Il veterinario sente un tumore sul gatto. Come sente? Sente con i mani.
Il veterinario lavora sodo. Come si sente? Stanco!
Things can't "feel"; when you ask someone "how does it feel" you're rather asking them how they feel about it. In Italian that's usually "cosa provi?" or "cosa si prova?".
Hm yes, that's true; it still doesn't feel right to ask how an animal is feeling though - there's no way to get an answer. "What" it feels (e.g. pain or pleasure) would be more likely.
No, "how does it feel" even in english means a completely different concept. The subject is "the thing being felt", not the "person who is feeling".
E.g.: "How does it feel to put your hand in a fire?"
like 'come si sente mettere la mano nel fuoco'? or should it be 'come si sente uno mettendo la mano nel fuoco'? for me, both would sound good.
The first one presents 2 problems:
1 - There is no subject, and "sentirsi" is already reflexive. So you have to make impersonal a verb that already contains the particle "si". You have to use an additional "ci" for this purpose. Then, it becomes "Come CI si sente?"
2 - When verbs are used as complements and follow the main verb, you have to leave the "to" in the infinitive. Remember you say "how does it feel to put...?", in english, not "how does it feel put...?". Italian "to" is "a".
Putting all together, your first sentence becomes "Come CI si sente A mettere la mano nel fuoco?".
'come si sente uno mettendo la mano nel fuoco'? is ok, you could say "come si sente uno a mettere la mano nel fuoco?", alternatively.
could this mean "how does one hear" (eg if you were trying to talk to someone at at loud concert)
Hm no, it wouldn't feel right; you could say it when testing the mic though (kind of "how well can you hear?").
Io pensavo a un impianto audio e l'ho sbagliata. Giuro son madrelingua