In syntax, a sentence with an independent clause and at least one dependent clause is referred to as a complex sentence. The dependent clause is often introduced by a subordinate conjunction such as "which", "while" or "because".

Examples

• I ate the meal which you cooked.

I ate the meal is an independent clause, which you cooked is dependent.

• When I saw what you had done, I was happy.

Two dependent clauses, and one indepedent.

A sentence with a relative clause like the first example does not count as a complex sentence. A sentence is complex only when it contains a subordinate clause which fulfils a syntactic function within the sentence. In the second example, the sub-clause "When I saw what you had done" functions as adverbial, it has a temporal meaning. The sub-clause "what you had done" is embedded in the first subclause and functios as direct object within it. (See is a transitive verb and takes a direct object) A clause may also function as subject, or subject complement.

A relative clause has no function within the sentence, but is embedded in a noun phrase and only describes this noun phrase, thus it does not built a complex sentence.

Example for a complex sentence with a sub-clause functioning as subject: That you love me makes me happy.

Example for a complex sentence with a sub-clause functioning as exraposed subject: It makes me happy that you love me. "It" (dummy-it) does not have a meaning, it only takes the position of the subject, but the real subject occurs at the end of the sentence.

Example for a complex sentence with a sub-clause functioning as subject complement: The book is where you have put it. "Be" is a copula verb, it links the sub-clause to the subject.

Contrast with:

• I was scared, but I didn't run away.

Both clauses are independent. This is a compound sentence but not a complex sentence.

• The dog you gave me barked at me and bit my hand.

Two independent clauses ("barked at me" and "bit my hand") and one dependent clause ("[which] you gave me"). This is a complex-compound sentence .