The complex sentence is made up of a main idea (or independent clause) and one or more additional clauses (subordinate or dependent clause) that provide more information. They are joined by subordinating conjunctions. Example: I didn’t see you at the movie because we sat right in front.
The subordinate clauses give explanations or more information about the main, independent clause.
So the main idea is “I didn’t see you at the movie” and the dependent or subordinate clause (which gives an explanation) is “because we sat right in front”. “Because” is a subordinating conjunction.
Other examples of common subordinating conjunctions are: “after”, “although”, “before”, “if”, “since, “unless”, “until”, “when”, “whereas”, “while”.
When you are analysing sentences, the first thing to do is to find the complete or finite verbs, and then work out if they form the main independent clause, or pieces of sentence that give more information.