Who, whom and whose are relative pronouns. Who and whom are used for persons. They introduce relative clauses postmodifying the head of a noun phrase, and they are identical in form with interrogative pronouns but function differently.

Who: nominative form.
"The student who failed the test was very upset."
"I know the address of the girl who forgot her keys on my desk."

Whom: objective form.
Whom is formal and used mainly in writing. In conversation it is usually replaced by who (unless it follows a preposition):
"Tom, who(m) I have always trusted, told me the truth about the incident."
"The beggar to whom I gave some bread last night is back."
or
"The beggar who(m) I gave some bread to last night is back."

Who(m) may be omitted as objects in a relative clause:
"Tom is a man [whom] I have always trusted."
"The beggar [whom] I gave some bread to last night is back."

Whose: possessive form.
"That is the teacher whose students are bright."
"The new book, whose aim is to introduce you to the world of linguistics, is now available at the library."

Here are a few examples of interrogative pronouns:
"Who was at the party last night?"
"To whom did you lend my books?" / "Who(m) did you lend my books to?"
"Who(m) will you invite to your birthday party?"
"Whose notebook is this?" / "Whose is this notebook?"

If you can understand this, you could try to complete the sentences you posted. Give it a try!

Miriam

1 is ok (I’d use “stamp-collecting” or “collecting stamps”, though).
2 has a “to” too many. you can choose which one you wish to get rid of.
“Can I ask who(m) I’m talking to?”
or
“Can I ask to whom I’m talking?”
3 is ok

“My dog whose fur is back is coming” is grammatically correct, although I’m not quite sure what it means. It sounds a bit ambiguous to me.

All the sentences in the previous posts contain relative clauses (adjectival clauses). It is important to distinguish between restrictive (or defining) and non-restrictive (or non-defining relative clauses).

The use of commas makes a difference in meaning between:
a) “My friend, whose hobby is collecting stamps, is coming"
and
b) “My friend whose hobby is collecting stamps is coming”
The first example implies that you have only one friend, and that they are coming to see you. The second implies that you may have several friends but only one of them –the one whose hobby is collecting stamps- is coming.

We call the relative clause in a) “non-restrictive”. A non-restrictive relative clause appears between commas and if it is omitted the sentence will still make perfect sense. The information provided by a non-restrictive relative clause is additional and not necessary for the sentence to make sense, it does not define the antecedent (the head of the nominal clause). This type of relative clause could be written between parenthesis or dashes.

There is yet another type of non-restrictive relative clause in which the antecedent is the whole clause/sentence. This type is always introduced by “which”, and the sentence it modifies may be replaced by “the fact”:
“He missed the train, which annoyed him very much”
It was missing the train (the fact that he mised the train) which annoyed him. “which” is replacing the whole clause, not “train”.

The clause in b) is “restrictive”. A restrictive relative clauses does not appear between commas, and the information it provides is necessary for the sentence to make complete sense. This type or relative calsue defines the “antecedent” (the head of the nominal phrase) and give it its definite connotation. In the example, we are told which of his brothers lives in London.

You also asked about “which”, “that”, “this” and “those”.
This/that and their plurals, these/those are demonstratives, and they may finction as either adjectives or pronouns.
Adjectives:
“This book is mine.”
“Those houses over there belong to the owner of the company.”
Pronouns:
“This has never happened to me before.”
“That is not your car.”
“Those are my sister’s books.”

“That” is sometimes confusing to students because the same spelling corresponds to different different parts of speech and fulfil different functions in a sentence. Two of them we have just seen (adjective and pronoun).
“That” is also a “relative pronoun”, and this is the type that can introduce a relative clause.
“The man that I met last week lives in Manchester.”

There is so much more to say about relative clauses! Let me take a break and I’ll continue later.
By the way, do you happen to have a grammar book? All this is surely much better explained in a book, more clearly and with lots of examples.

I hope this won't confuse you more

Miriam