A complement completes the verb. It’s similar the subject because it’s usually a noun or noun phrase; however, it generally follows the verb when the sentence is in the active. Note: Every sentence doesn’t require a compliment. The complement cannot begin with a preposition. The complement answers the question what? Or whom?
Example 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 Phea at the movies last night. (Whom did she see at the movies?)
They called Robert yesterday. (Whom did they call yesterday?)
He was smoking a cigarette. (What was he smoking?)