The _simple sentence_ expresses a single thought and consists of one
subject and one predicate, as, "Man is mortal."

A _compound sentence_ consists of two or more simple sentences of equal
importance the parts of which are either expressed or understood, as,
"The men work in the fields and the women work in the household," or "The
men work in the fields and the women in the household" or "The men and
women work in the fields and in the household."

A _complex sentence_ consists of two or more simple sentences so combined
that one depends on the other to complete its meaning; as; "When he
returns, I shall go on my vacation." Here the words, "when he returns"
are dependent on the rest of the sentence for their meaning.

A _clause_ is a separate part of a complex sentence, as "when he returns"
in the last example.

A _phrase_ consists of two or more words without a finite verb.

Without a finite verb we cannot affirm anything or convey an idea,
therefore we can have no sentence.

Infinitives and participles which are the infinite parts of the verb
cannot be predicates. "I looking up the street" is not a sentence, for it
is not a complete action expressed. When we hear such an expression as "A
dog running along the street," we wait for something more to be added,
something more affirmed about the dog, whether he bit or barked or fell
dead or was run over.

Thus in every sentence there must be a finite verb to limit the subject.