[quote=Unregistered;409437]Please, explain me the difference between "who" and "that" and correct my mistakes.
1. You shouldn't live with a man .who./.that. doesn't love you.
2. I called my friend, ..who.. is a dentist, to fix my teeth, ..all which..decayed.
3. Poetry is a language..which.. says more than the ordinary words can.
4. The man .that./.who. stole the money hasn't been found yet.
5. This is the house .whose. walls were repainted several times.
6. I found a way ..by which.. I can go home earlier.
7. He is having lunch, ..which.. he always does at this time.
-Used for personal reference (human beings) or things that can be thought of as entities resembling humans, because the speaker has attributed human-like qualities to them (eg: My dog, who is my best friend,...).
-Can be used both in defining and non-defining relative clauses.
-Normally, and in contrast to 'who', can be used both for personal and non-personal reference (as in '1.' and '4.').
-Is more informal than 'who'.
-Cannot normally be used in non-restrictive/non-defining relative clauses.
-cannot be used after a preposition (as in '6.').
Non-defining relative clauses are the ones that introduce 'non-essential' information, and are usually between commas.
"2. I called my friend, ..who.. is a dentist, to fix my teeth, ..all which..decayed."
As you are singling out your friend by saying who of them you are talking about (the one who is a dentist), in my opinion, this should'nt be between commas (because this information is essential to the meaning of the sentence).