What is a Sentence?

In simple terms, a sentence  is a set of words that contain:

1. a subject  (what the sentence is about, the topic of the sentence), and
2. a predicate (what is said about the subject)

Look at this simple example:

The above example sentence is very short. Of course, a sentence can be longer and more complicated, but basically there is always a subject and a predicate. Look at this longer example:

Note that the predicate always  contains a verb. Sometimes, in fact, the predicate is only a verb:

So we can say that a sentence must contain at least a subject and verb.

There is one apparent exception to this – the imperative . When someone gives a command (the imperative), they usually do not use a subject. They don't say the subject because it is obvious - the subject is YOU! Look at these examples of the imperative, with and without a subject:

Note that a sentence expresses a complete thought. Here are some examples of complete and incomplete thoughts:

Note also that a sentence begins  with a capital letter and ends  with a full stop (AmE  period) or a question mark or an exclamation mark (AmE  exclamation point). Look at these examples:

• People need food.
• How are you?
• Look out!

sources : Original Link