A complex sentence is a type of sentence that contains one independent clause and at least one dependent clause. The independent clause can stand alone as a complete thought, while the dependent clause cannot. For example, in the sentence "Although it was raining, we went for a walk," "we went for a walk" is the independent clause, and "Although it was raining" is the dependent clause. Complex sentences often use subordinating conjunctions like although, because, or since to connect the clauses. This structure allows for more detailed and nuanced expressions of ideas, making writing richer and more informative. Using complex sentences can enhance clarity and depth in communication.