Apostrophes are used to show possession (something belonging to someone or something) and also for contractions (the shortening of two words into one).

When you use apostrophes think of the process like a flowchart. Do I want to show possession, or contract two words into one?

You have several options:

Possession = before the s, or after the s

Contraction = remove one letter, or two (something more) letters.

Have a look at the flowchart picture in the next lesson! You also have this attached under course materials.