A SENTENCE is a group of words that contains a subject, a verb, and expresses a complete thought or idea.

In order to find out if your group of words is a sentence, find the action word (verb) and then ask yourself "who or what" did the verb.

Examples:

The dog will eat all of his food.

"Who or what will eat his food?"

 (dog is the subject, will eat is the verb)

Many children study grammar in school.

"Who or what study grammar in school?"

(children is the subject, study is the verb)

You must practice is a skill in order to get better at it.

"Who or what must practice a skill in order to get better?"

(You is the subject, must practice is the verb)

If your group of words is missing a subject or verb, or it does not express a complete thought, then it is considered a fragment.

Examples:

Tommy the tall boy.

This word group has no verb.

In order to get better at it.

This word group has no subject or verb.

Running and yelling all the time.

This word group has no subject.