This is one of the most common problem areas of our language, probably because possessives almost always use apostrophes. It's time to go. It's been great. Often used incorrectly, but don't get nauseating about its usage. Pregnant women often experience nausea.
Timing is everything.
This is probably the second most common problem area in our language. Someone said you're leaving. Tom, thanks for giving your time to this effort. Follow the principles of a good life. Now try a few of these exercises to see which of the common errors can be corrected.