"Their" is best described as a possessive determiner, a category often taught
as a type of adjective or possessive pronoun. Its function is to modify a
noun by showing ownership.
Here's a breakdown of the different classifications:
Possessive Determiner: This is the most precise grammatical term. Determiners
are a class of words (like the, a, some, and this) that introduce noun
phrases and specify or limit the noun's meaning. Since "their" indicates
possession, it belongs to the subcategory of possessive determiners.
Possessive Adjective: In traditional grammar, "their" is called a possessive
adjective because it comes before a noun and describes it, just like other
adjectives. For example, in the phrase "their new car," "their" modifies the
noun "car".
Possessive Pronoun: Some sources categorize "their" as a possessive pronoun
because it is the possessive form of the pronoun "they". However, this can
cause confusion with the independent possessive pronoun "theirs," which
stands alone without a noun.
Their" vs. "Theirs"
To clarify the distinction between "their" and the possessive pronoun
"theirs," consider these examples:
Their (determiner/adjective):
"The children are playing with their toys".
"They brought their own food".
Theirs (independent pronoun):
"The toys on the floor are theirs".
"That food is theirs, not mine".