That is known as the RESTRICTIVE RELATIVE, because it should be used whenever the relative clause limits the substantive, unless who or which is of more pleasing sound in the sentence. In the sentence, He is the man THAT DID THE ACT, the relative clause, that did the act, defines what is meant by man; without the relative clause the sentence clearly would be incomplete. Similarly, in the sentence, The book THAT I WANT is that red-backed history, the restrictive relative clause is, that I want, and limits the application of book.
Who and which are known as the EXPLANATORY or NON-RESTRICTIVE RELATIVES, and should be used ordinarily only to introduce relative clauses which add some new thought to the author's principal thought.
Spanish, WHICH IS THE LEAST COMPLEX LANGUAGE, is the easiest to learn. In this sentence the principal thought is, Spanish is the easiest language to learn. The relative clause, which is the least complex language, is a thought, which, though not fully so important as the principal thought, is more nearly coordinate than subordinate in its value. It adds an additional thought of the speaker explaining the character of the Spanish language. When who and which are thus used as explanatory relatives, we see that the relative clause may be omitted without making the sentence incomplete.
Compare the following sentences:
Explanatory relative clause: That book, which is about history, has a red cover.
Restrictive relative clause: The book that is about history has a red cover.
Explanatory relative clause: Lincoln, who was one of the world's greatest men, was killed by Booth.
Restrictive relative clause: The Lincoln that was killed by Booth was one of the world's greatest men.
An Interrogative Pronoun is a pronoun used to ask a question. The interrogative pronouns are who (whose, whom), which, and what. In respect to antecedents, who should be used only in reference to persons; which and what may be used with any antecedent, persons, animals, or things.