' How to use the apostrophe

Rules of punctuation made simple.

The apostrophe indicates contraction

Alice can not sing --> Alice can't sing

Alice will not sing --> Alice won't sing

Alice shall not sing --> Alice shan't sing

Alice must not sing --> Alice mustn't sing

The apostrophe takes the place of the missing letter or letters.

The apostrophe indicates posession

To show singular posession, add: 's

The apple belonging to Carol --> Carol's apple

The grades of one student --> The student's grades

To show plural posession, add an apostrophe at the end: '

The grades of many students --> The students' grades

A final rule: If in doubt, leave it out.
If you forget the apostrophe, it won't be noticed. It is a minor error in punctuation.
On the other hand, if you misuse the apostrophe, it will be noticed.

Better to remain silent, and be thought a fool, than to open one's mouth and remove all doubt.