Apostrophes in English have many rules, along with exceptions to these rules. Anyway, their two main uses are:

1] To show that some letters have been taken out of a word to shorten it.

The apostrophe goes where the letters have been removed, e. g.

• is not becomes isn‘t
• do not > don’t
• I would like to > I‘d like to

2] To show belonging.

To do this, we usually add ’s.

• the horse’s tail – shows that the tail belongs to the horse
• Paul‘s bike is …
• Jen‘s brother works as …

Exception

In words that end in -s apostrophe goes after the -s that is already there, e. g.:

• The octopus’ legs are thin = the legs belong to the octopus