When is an adjective clause essential?

A adjective clause is a is a dependent clause used as an adjective. Most of the time it begins with pronoun for a example which.

So it would most be essential when your "looking at a sentence that starts with a pronoun (most likely in a short story) and it sounds more of a question. It would also be a dependent clause and would function as an adjective as in answering questions for example: Which one?"

Hope this helps!