Examples:
* The book which I read yesterday was very interesting. ("Which" modifies "book" and introduces the clause "which I read yesterday.")
* The car, which is red, belongs to my neighbor. ("Which" modifies "car" and introduces the clause "which is red.")
However, "which" can sometimes appear in a phrase that functions as an adjective.
Example:
* The which way should we go? ("Which way" acts as an adjective phrase modifying "way".)
In this case, "which" is part of a larger phrase that acts as an adjective, not an adjective on its own.