Even native English speakers have a hard time using apostrophes correctly. I don’t see why this should be, since the rules are fairly simple, but it is so. In fact, I’ll confidently say I’ve seen more correct use of apostrophes in English by Germans than by most native English speakers. Nevertheless, there’s still some confusion even among detail-oriented Germans.
Fortunately, Bob the Angry Flower has a quick chart explaining when you do and when you don’t use an apostrophe in English: