I always had trouble with lie and lay. In fact when I was a freshman in college, a friend from Brazil who was a Library Science major at Simmon's college across the street asked me to explain it to her one day, and I said, "I can't! Call my mother, she'll tell you!"

Now I can always look it up - http://grammartips.homestead.com/lie.html is one example. Now can you imagine trying to explain "intransitive verb" to someone just learning English?