Sentence: A complete thought, usually sayable in one breath.

When you say a sentence, it is of four purposes:

Declarative asserts truth or falsehood.

Interrogative asks for truth or falsehood.

Imperative commands or requests.

Optative wishes for the non-existent.

(Some argue that the exclamatory is a purpose, but note that you can speak any of the sentences plainly or with force.)

The sentence ends in three ways:

Period. Question mark? Exclamation mark!

(In this age of slapping down punctuations with no regard for rules, do not think this is an unnecessary review.)

The sentence is left incomplete in two ways:

The long dash interrupts thought.

The ellipses leaves the thought purposefully.

If you would like, you can teach this with two fists raised, and make one hand the sentence purpose/type, and the other hand the ways to end the sentence. Five and five. See it?