Relative Pronouns in Restrictive Relative Clauses

Introduction and General Usage in Defining Clauses

The most common relative pronouns are who/whom, whoever/whomever, whose, that, and which. (Please note that in certain situations, "what," "when," and "where" can function as relative pronouns.)
Relative pronouns introduce relative clauses, which connect adjectival and adverbial dependent clauses to a main clause. Adjectival and adverbial dependent clauses modify a word, phrase, or idea in the main clause. The word, phrase, or idea modified is called the antecedent. In the following examples, that and whom modify the subject:

The house that Jack built is large.

The professor, whom I respect, recently received tenure.

The type of clause determines what kind of relative pronoun to use. Generally, there are two types of relative clauses: restrictive (defining) clause and non-restrictive (non-defining) clause. In both types of clauses, the relative pronoun can function as a subject, an object, or a possessive pronoun ("whose").

Relative Pronouns in Restrictive Relative Clauses

Relative pronouns that introduce a restrictive relative clause ARE NOT separated from the main clause by a comma. Restrictive relative clauses (also known as defining relative clauses) add essential information about the antecedent in the main clause. The information is crucial for understanding the sentence's meaning correctly and cannot be omitted. In other words, without the restrictive relative clause, the sentence does not make sense.
The table below sums up the use of relative pronouns in restrictive relative clauses:

Function in the sentence	Reference to
	People	Things / concepts	Place	Time	Explanation
Subject	who, that	which, that
Object	(that, who, whom)*	(which, that)*	where	when	what/why
Possessive	whose	whose, of which

Examples

Relative pronouns used as a subject of a restrictive relative clause:

This is the house that had a great Christmas decoration.

It took me a while to get used to people who eat popcorn during the movie.

Relative pronouns used as an object in a restrictive relative clause:
1) As can be seen from the table, referring to a person or thing, the relative pronoun may be omitted in the object position, but formal English includes the relative pronoun. When the relative pronoun is the object of a preposition, which is used instead of that, for example, "in which," "for which," "about which," "through which," etc. (please see the third example below):

Formal English: This is the man to whom I wanted to speak  and whose name I had forgotten.
Informal English: This is the man (whom/that) I wanted to speak and whose name I'd forgotten.

Formal English: The library did not have the book that I wanted.
Informal English: The library didn't have the book I wanted.

Formal English: This is the house where/in which I lived when I first came to the US.
Informal English: This is the house I lived in when I first came to the US.