"Birds fly;" "Fish swim;" "Men walk;"—are sentences.
A sentence always contains two parts, something spoken about and something said about it. The word or words indicating what is spoken about form what is called the subject and the word or words indicating what is said about it form what is called the predicate.
In the sentences given, birds, fish and men are the subjects, while fly, swim and walk are the predicates.
There are three kinds of sentences, simple, compound and complex.
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.