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:

sentence
subjectpredicate
verb 
YouspeakEnglish.

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:

sentence
subjectpredicate
verb 
Ram and TaraspeakEnglish when they are working.

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

sentence
subjectpredicate
verb 
Smokerises. 

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

There is one apparent exception to this ‚Äď 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:

sentence
subjectpredicate
verb 
 Stop! 
 Waita minute!
Youlook! 
Everybodylook! 

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

  complete thought?
sentenceHe opened the door.YES
Come in, please.
Do you like coffee?
not a sentencepeople who work hardNO
a fast-moving animal with big ears

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!
Actually, it is not easy to define a sentence. Grammarians do not all agree on what is or is not a sentence. For the purposes of introduction, this page describes rather simple sentences. Of course, sentences can be much longer and more complex, and these will be covered on other pages.

sources : Original Link