A complex sentence has an independent clause joined by one or more dependent or subordinate clauses.  We want to express two or more ideas in one sentence. A dependent clause cannot stand alone even though it has a subject and a verb.

as, as if, before, after, because, though, even though, while, when, whenever, if, during, as soon as, as long as, since, until, unless, where, and wherever –  a complex sentence is often formed by putting the aforementioned words, called subordinating conjunctions, at the beginning of the dependent clause:  A complex sentence can also use a relative pronoun such as that, who, or which.

examples of a complex sentence

If a million people say a foolish thing, it is still a foolish thing. Anatole France [Jacques Anatole Thibault] Note that the clause – “If a million people say a foolish thing” cannot stand alone and is therefore a dependent or subordinate clause. “It is a foolish thing.” is an independent clause joined to the dependent clause by a subordinating conjunction – if.
I went to the doctor because I cut my toe.
I will be able to dance if my toe heals by then.
Once they had completed their exam, they went surfing.
The man who visited us was selling vegetables. Though the vegetables were fresh, we did not buy any.

When the subordinating conjunction is at the beginning of the sentence – “Though the vegetables were fresh, we did not by any.”, the reader has to decide which idea is the more important one. By using ‘though’ in the beginning of the sentence, you are saying that, although the vegetables were fresh, the main point is that you did not buy them.  Note that with a complex sentence beginning with a subordinator a comma is required at the end of the dependent clause. When the independent clause begins the sentence with subordinators in the middle, no comma is required.

Those complex sentences which begin with a dependent clause and end with an independent clause are known as periodic sentences. They are more effective in conveying the thought as the first part of the sentence forms the meaning which becomes complete by the end.  A complex sentence makes clear which idea is most important, and at the same time it adds interest to the sentence