In "Flowers for Algernon," how does Charlie's intellect change throughout the novel? (Please use quotes.)

Charlie goes from having a very low IQ to a very high IQ. He then reverts to his low IQ.
Charlie, not knowing what he is getting into, agrees to an operation that might make him more intelligent. He writes that he is told that by participating in the "experament you mite get smart. They dont know if it will be perminint but theirs a chance. Thats why I said ok even when I was scared because she said it was an operashun."
We are also informed that Charlie's IQ is 68:

I felt proud when he said that not every body with an eye-q of 68 had that thing [motivation].

Charlie's increase in intelligence is reflected in his improved grammar, spelling, and vocabulary. We also see evidence for his increased intelligence in how quickly he learns to type and in how fast he can type:

Dr. Strauss is very angry at me for not having written any progress reports in two weeks. He's justified because the lab is now paying me a regular salary. I told him I was too busy thinking and reading. When I pointed out that writing was such a slow process that it made me impatient with my poor handwriting, he suggested that I learn to type. It's much easier to write now because I can type nearly seventy-five words a minute. Dr. Strauss continually reminds me of the need to speak and write simply so that people will be able to understand me.

The experiment fails, and Charlies reverts back to the intelligence he began with. This is heartbreaking because he now has the intellect to know what is happening and is going to happen to him. He writes:

I've given up using the typewriter completely. My co-ordination is bad. I feel that I'm moving slower and slower. Had a terrible shock today. I picked up a copy of an article I used in my research, Krueger's Uber psychische Ganzheit, to see if it would help me understand what I had done. First I thought there was something wrong with my eyes. Then I realized I could no longer read German.

At the end, his writing reflects his return to his old state:

Evry body feels sorry at the factery and I dont want that eather so 1m going someplace where nobody knows that Charlie Gordon was once a genus and now he cant even reed a book or rite good.