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 are useful for adding detail and depth to writing. They allow writers to express relationships between ideas, such as cause and effect or contrast. By using complex sentences, writers can create more nuanced and sophisticated expressions of thought, enhancing clarity and engagement in their communication.