Incorrect use of the apostrophe is very common. We see it on TV, on road signs and in the newspapers. Most common is the incorrect use of its/it's. For some examples, see here.

Maybe apostrophes are more trouble than they're worth. But they exist and cause students to lose marks in exams.

So let's give the rules for their use in the simplest possible way.

Frequently both ESOL and first-language speakers of English make mistakes with contractions, the shortened forms of work combinations like "I'm" for "I am". The following web-site has a 50-item exercise which can give you practice and instant feedback although it doesn't give you the actual answers. Go to Web School. There are several other items at the Web School too. Their home page is here; click on the "Language" option and then "Daily Language".