Explain three instances in which apostrophes are used to indicate omission; provide an example for each instance.

They can be used to show omission of numbers, like in a year. For example, instead of writing "2017" you could say '17.

They are used to show omission in a contraction. So, instead of saying "let us" you would say "let's"

Finally, it's used to make words shorter. For example, instead of saying "because" you could say " 'cause "