Theory:

Sentences generally begin with capital letter and ends in a full stop/question mark/exclamation mark.
Sentences are of four types.
  1. Statement - Facts or opinions.
  2. Questions.
  3. Instructions/commands/requests.
  4. Exclamation - Strong emotions like wonder/surprise/anger etc.
Lets take a look at the picture for a clear explanation and example.
 
Sentences.png