Complex Sentences

A complex sentence contains one independent clause — a clause that can stand on its own — and at least one dependent clause, which cannot. The dependent clause is attached to the independent clause by a subordinating conjunction, and this relationship of dependency is what gives the complex sentence its name. Where a compound sentence balances equal ideas, a complex sentence creates hierarchy: one idea is the main point, and the other provides context, reason, condition, or qualification.

Subordinating conjunctions are the gateway to complex sentences. The most common include after, although, as, because, before, even if, if, since, though, unless, until, when, where, and while. Each one signals a specific logical relationship: because introduces a reason, although a concession, when a time condition, if a hypothetical. The choice of subordinator shapes how the reader understands the connection between the two clauses — swap because for although and the entire logical structure of the sentence flips.

Punctuation in complex sentences follows a clear rule tied to clause order. When the dependent clause comes first, place a comma between it and the independent clause: Because it was raining, we stayed inside. Even if he doesn't study, he's going to pass his test. This comma signals to the reader that the introductory clause has ended and the main clause is about to begin. When the independent clause comes first, no comma is needed: We stayed inside because it was raining. The subordinating conjunction itself does the work of connecting the clauses.

A persistent myth says you cannot begin a sentence with because. This is false — as long as the because-clause is attached to an independent clause. Because I was tired, I went to bed is perfectly correct. The error would be leaving the dependent clause alone: Because I was tired. That is a sentence fragment, not a complex sentence. The same principle applies to any subordinating conjunction: a dependent clause standing alone is always a fragment.

Dependent clauses serve three main functions within a complex sentence. Adverbial clauses modify the verb of the main clause, telling when, why, how, or under what condition: I'll call you when I arrive. Adjectival clauses (also called relative clauses) modify a noun: The woman who lives next door is a surgeon. Nominal clauses act as a noun — as the subject, object, or complement of the main clause: What you said surprised everyone, I don't know where he went. Mastering these three types gives you enormous flexibility to express relationships of time, cause, condition, description, and identity within a single sentence.