An adjective clause, also called, a relative clause, starts with a RELATIVE PRONOUN, such as who, which, or that. Or an adjective clause can start with a RELATIVE ADVERB, such as when or where.
An adjective clause modifies the NOUN or PRONOUN that it follows:
A.) The car that Jack bought is practical. [The adjective clause describes the noun car; that is a relative pronoun referring to car.]
B.) The day when I can buy my own car is getting closer. [The adjective clause modifies the noun day; when is a relative adverb referring to day.]
Use who, whom, whoever, whomever, and whose when an adjective clause refers to a person or to an animal with a name:
A.) The Smythes, who collect cars, are wealthy.
B.) Their dog Bowser, who is large and loud, has been spoiled.
Use which or that when an adjective clause refers to a thing or to an animal that isn't a pet. Sometimes writers omit that from an adjective clause. For grammatical analysis, however, consider the omitted that to be implied and, therefore, present.