A complex sentence is one in English in which includes, at minimum, one dependent clause and one independent clause. The dependent clause, which is also known as a subordinate clause, is connected by either a relative pronoun or a subordinate conjunction. The complex sentence can be contrasted with a simple sentence, which has no dependent clause. Other types of sentences include the compound sentence, which need to have at least two independent clauses, and the complex-compound sentence, which need to have at least two independent clauses, and at least one dependent clause.
An independent clause, one of the two critical parts of a complex sentence, is on its own a simple sentence. It is a clause which is complete in-and-of itself, with both a subject and a predicate. For example, the sentence, I like to write letters, is an independent clause. Two independent clauses may be linked together to form a compound sentence, as in, I like to write letters, and she sure likes to tango. In the compound sentence, two individual sentences, I like to write letters. and She sure likes to tango. are connected via a coordinating conjunction, and.
A dependent clause may also be known as a subordinate clause or an embedded clause, and is a clause that cannot grammatically stand alone. It should be noted that strictly speaking a subordinate clause should refer only to an adverbial dependent clause. A dependent clause, unlike an independent clause, is not complete by itself: it needs something to complete it. For example, the clause, She is, is not really complete by itself, even though it contains a subject and a predicate. To be complete, we could add a subordinating conjunction, like whoever, and link it to an independent clause. For example, the sentence Whoever she is, she sure likes to tango. is a complete complex sentence.
Aside from the simple subordinate conjunction example above, many different structures can make up the complex sentence. For example, the dependent clause may act as the subject in a complex sentence. Look at the dependent clause, How you live, which cannot stand alone. Then look at the independent clause A good job makes all the difference. In this independent clause, the subject, a good job can be replaced with many different subjects, giving us sentences like: Money makes all the difference. or The first kiss makes all the difference. We can also replace the subject with our earlier dependent clause, giving us the complex sentence: How you live makes all the difference.
A complex sentence may also include an independent clause that has been split by a dependent clause. For example, the sentence, The chair is old. is an independent clause. The clause, John is sitting in is a dependent clause, unable to sit on its own. By using a subordinator, we can put our dependent clause in the middle of our independent clause to create a complex sentence, creating: The chair that John is sitting in is old.
It’s good to notice that the order in which the dependent and independent clauses occur has no bearing on whether the sentence of complex or not, or whether the clause is independent or dependent. For example, the sentence, After the rain finally stopped, we went to the beach. is a complex sentence in which the dependent clause comes first, and the independent clause comes second. That same sentence can be flipped around to form, We went to the beach after the rain finally stopped, without changing the meaning of the sentence at all, or how the two clauses relate to one another.