Complex sentences always contain an independent clause and a dependent clause. All clauses have a subject and a verb. An independent clause can be a sentence by itself. A dependent clause needs to be connected to an independent clause to make sense. Here are several examples of complex sentences. The dependent clause in each sentence is underlined. You can see that each dependent clause has a subject and a verb, but it would not be a clear statement if it were not connected to an independent clause.
Independent clauses are also known as main clauses.
Dependent clauses are also known as subordinate clauses.
The terms "main" and subordinate suggest the role that each of these types of clauses play in the structure of complex sentences.
The main clause carries the main idea of the sentence, while the subordinate clause supports that main idea in some way. The subordinate clause can function as a modifier, like an adverb or adjective that describes something in the main clause. The subordinate clause can also function as a noun. The entire clause can be a subject or an object in the main clause.
We can see how this works by taking another look at the three complex sentences that we saw above.