A sentence is a group of words which expresses a complete thought.

A wolf lives -- This is not a sentence because it does not express a complete thought.

A wolf lives in a group called a pack. -- This is a sentence because it expresses a complete thought.

4 Types of Sentences

Declarative - sentence that makes a statement and ends in a period.

An orca is a member of the dolphin family.

Interrogative - sentence that asks a question and ends in a question mark.

Have you finished your homework?

Imperative - sentence that gives a command or makes a request and ends in a period.

Kathy, go clean your room.

Exclamatory - sentence that expresses strong feelings or emotions and end in an exclamation point.

Green Bay won the game!