The boy whose mother died is

The name and function of what? Surely not the entire sentence!

The phrase "whose mother died" is a relative clause. It functions as an adjective clause describing the noun "boy." The word "whose" is a possessive pronoun introducing the clause, and the verb "died" is the main verb of the clause.

The phrase "whose mother died" in the given sentence is called a relative clause. The noun phrase "the boy" is modified by the relative pronoun "whose," which connects the clause to the noun it is referring to.

In terms of function, this relative clause acts as an adjective clause because it provides additional information about the boy. Specifically, it specifies that the boy's mother has died.