An example of a complex sentence would be the following: Because I am allergic to dairy, I cannot eat cheese on my pizza. Complex sentences have one or more dependent clauses combined with an independent clause. In the example sentence, the subordinate clause begins with because.
Subordinators include: because, after, since, when, before, although, when and many others. Relative pronouns, such as who, that and which, also create complex sentences. When a dependent clause begins the sentence, a comma is necessary after the dependent clause, as is the case in this sentence. No comma is needed when the dependent clause follows the independent clause.