Ответ:

Relative pronouns are used to associate subordinate clauses with the principal. They are allied words, which differ from alliances in that they not only associate the subordinate clause with the principal, but are also members of the subordinate clause.

For communication between the subordinate clauses of the subject, predicate and complementary with the main one, pronouns are used: who who (whom whom), whose whose, what who, what, which which, what, who, what. They are thus the same interrogative pronouns, but not used for the question, but for the connection of sentences):

Who has done it is unknown. Who did it is unknown.

I do not know. I do not know which of them speak French.

That is not what I want. This is not what I want.

Note. The relative pronoun which is often translated into Russian by what.

For the connection of definitive subordinate clauses with the main pronouns are who with the meaning of which (whom), whose, which and that with the meaning of which, of which:

The man who is sitting next to Mr. A. is my English teacher. The person who sits next to Mr. A. is my English teacher.

The watch that I lost was a very good one. The hours I lost were very good.