How does Charlie understand commas in "Flowers for Algernon" by Daniel Keyes?

When Charlie first starts writing his progress reports for Dr. Strauss, he uses very little punctuation. In his April 16 progress report, he writes that Miss Kinnian taught him about the comma that day. He calls it a "period with a tail" (Keyes 14). Charlie knows commas are important, but he hasn't quite figured out where to use them as his progress report is filled with them. He thinks that all of the periods should have tails, but once Miss Kinnian corrects him, he begins to understand. Charlie also finds out there are many different kinds of punctuation marks. In his next progress report, he uses all of them, but he obviously still doesn't quite get it. However, on April 18, Charlie reads the entire grammar book, and once he does that, he completely understands all punctuation from periods and commas to dashes and apostrophes!