When a group of words forms a sentence, it expresses a complete thought. But when a group of words, such as a clause or phrase, that does not express a complete thought, it is a sentence fragment. A sentence fragment comes about when something – a subject, a verb or both – is missing in it. The presence of a subject and a verb can still give rise to a sentence fragment. More on sentence fragment.
A simple sentence has one independent clause with a subject and a verb. A sentence having a compound subject or a compound verb is still a simple sentence. A simple sentence can have a modifier such as an adjective or adverb. More on simple sentence.
A subject complement forms a part of a predicate of a sentence. It can be divided into:
Predicate nominative or predicate noun is always a single noun or a noun phrase, or a pronoun.
Predicate adjective is an adjective modifying the subject.
Both predicate noun and predicate adjective follow a linking verb and rename or describe the subject of a sentence.