Stilted, Complex
Charlie's writing style changes somewhat after he undergoes surgery—his words get bigger and his sentiments get more grandiose—but if you check out the sentence structure throughout the book, Charlie keeps coming back to the stilted, choppy sentences of his first progress reports. Check it:
That made me feel good. After the lesson I went down and played with Algernon. We don't race anymore. (40)
This stilted style packs a lot of dramatic punch. After all, we know a ton is going on in Charlie's head; plus he's just learning to articulate himself, and writing doesn't necessarily come easily right away. All this adds up to choppy sentences with some complex thoughts going on under the surface.
All those progress reports can be tough to follow, but they help establish unevenness as part of the story. While the reports start short and sweet, with Charlie writing faithfully every day, they get longer and more complex as he gets smarter. The later reports also stretch over days and even months, showing us just how long Charlie goes without writing. The last report is a prime example of the choppy style, as Charlie picks up and puts down the journal at random. There's no way to tell when he chooses to write, but that's just another way he exerts control over his mind—and the reader.