In the following sentence, which is the dependent clause?
Sofia won't be able to compete in the race because she wrecked her car.

A clause is a group of words containing a subject and a verb or verb phrase. An independent clause expresses a complete thought while a dependent clause does not.
Dependent clauses begin with words like: after, although, because, before, if, since, that, until, what, when, where, who, which, and why
The dependent clause, "because she wrecked her car," contains the subject "she" and the verb "wrecked." It does not express a complete thought.