A sentence is

1
A sentence is
A sentence is a complete thought.
A sentence is one unit of thought that should
be complete in sense and structure. A sentence
should be written with the correct punctuation
marks and word order.
One sentence is one thought or idea.
2
To check if you have written a complete
sentenceask yourself the following questions

• Does my sentence start with a capital letter?
• Does my sentence end with a full stop,
  exclamation mark or a question mark?
• Does my sentence include a main subject (the
  person or thing being written about)
• Does my sentence have a verb (a doing word e.g.
  cook, eat, write, study, speak, go, run,) or a
  being word e.g. am, is, was, are)