In grammar, an independent clauses are a set of words that are independent which means they do not need any other elements to stand, they express a complete idea and also they have at least one subject and one verb. This type of clauses differ from dependent clauses because dependent clauses need from other clauses to be complete and also, these clauses are linked to independent clauses by subordinating conjunctions such as because, when or while. This means in the case of the sentence "Trevor was sleeping when the doorbell rang" this sentence contains a dependent clause which is "when the doorbell rang" linked to the independent clause "Trevor was sleeping" that even without the dependent clause express a complete idea and contains a subject and a verb. Therefore, the independent clause in this sentence is "Trevor was sleeping".
