writing/editing paragraphs

I had Clay write two paragraphs about his day with Bobby. On the bright side, he wrote two pretty solid paragraphs much more quickly than before. Unfortunately, he went back to making some basic mistakes with his sentence structure. We then went through each sentence and talked about the multiple options of how to improve it. It was a good exercise and brought up many of the rules of grammar we've gone over. Some some reason he's really having a hard time the the articles "a" and "an". He understands the rule, but for some reason keeps making the mistake.