Sometimes, a sentence can have a subject and a verb and complete the thought. For example, Jerry slept. Jerry is the subject, and slept is the verb. No other words are needed to complete the thought.
However, sometimes, a verb needs more information to be complete. For example, Sue threw. This is a fragment because although it contains a subject (Sue) and a verb (threw), it does not express a complete thought. More information is needed. We can complete the sentence by adding a complement (the ball). Now, we have a complete sentence: Sue threw the ball.
A complement must complete the sentence by receiving the action of the verb, renaming the subject, or describing the subject. Here are some examples of complements:
Robyn is my sister. (renames the subject)
Robyn sells pizza in her shop. (receives the action of the verb)
She is funny. (describes the subject)
Note: a prepositional phrase does NOT count as a complement. For example, Jerry slept on his bed. Jerry is the subject, slept is the verb, and on the bed is a prepositional phrase. The verb is still intransitive. An adverb also does NOT count as a complement. For example, Jerry rubbed sleepily at his eyes. Jerry is the subject, rubbed is the verb, and at his eyes is the prepositional phrase. Sleepily is an adverb, so it does not receive the action of the verb or rename or describe the subject.
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Click here to determine the difference between Predicate Nouns and Predicate Adjectives