A complement completes the verb. It is similar to the subject because it is usually a noun or noun phrase; however, it generally follows the verb when the sentence is in the active voice. NOTE: Every sentence does not require a complement. The complement CANNOT begin with a preposition. A complement answers the question what? Or whom?
Examples of complements:
John bought a cake yesterday. (What did John buy?)
Jill was driving a new car. (What was Jill driving?)
He wants to drink some water. (What does he want to drink?)
She saw John at the movies last night. (What did she see at the movies?)
They called Mary yesterday. (Whom did they call yesterday?)
He was smoking a cigarette. (What was he smoking?)