My students always had a really hard time understanding the use of apostrophes, both in making contractions and in possessive tense. I had to use little drawings and stories to drive the learning home in a way the children would remember.
Something about this process worked so well with these first graders that before very long, most of the class was on their SIXTH list of 100 words! By the end of the year, in the last week or so of the school year, I found myself making an EIGHTH list of 100 words for three of my kids.