Does Charlie fully understand that the surgery's effects will be temporary? Why or why not?
Charlie's diary entry on March 6th suggests that he doesn't fully understand that the surgery's effects could be temporary.
In that entry, notice how Charlie writes down much of the conversation between Professor Nemur and Dr. Strauss as they are arguing about whether or not to select Charlie for the operation--and yet Charlie barely understands the meaning of their words. He uses asterisks in place of letters and words he couldn't catch, as in "Charlie is not what you had in mind as the frist of your new breed of intelek** couldnt get the word *** superman." Significantly, there's nothing wrong with Charlie's hearing: the reason he can't process the words he hears is that he doesn't know them.
Take a close look, too, at how Charlie reports Professor Nemur's warning about why Charlie shouldn't get his hopes up too high about the outcome of the operation:
"He said Charlie we werked on this for a long time but only on animils like Algernon. We are sure thers no fisical danger for you but there are other things we cant tell until we try it. I want you to understand this mite fale and then nothing woud happen at all. Or it mite even sucseed temperary and leeve you werse off then you are now. Do you understand what that meens."
Here, readers notice that Charlie didn't use a question mark at the end of Professor Nemur's question, indicating that he probably didn't understand that he was being asked something. And Charlie doesn't say anything in response to Professor Nemur, even though the professor is clearly expecting Charlie to say something, like "Yes, I understand." Further, as the diary entry concludes, Charlie writes nothing to show that he has comprehended the professor's warning at all.
Why can't he understand the risk involved? Charlie has an IQ of 68, meaning his intelligence is very low. He struggles with understanding adults' conversations, and he's essentially a beginner in reading and writing, as his spelling in the quotes above reveal. So he simply doesn't mentally process the risk involved in the operation. He's also so thrilled about the possibility of becoming smarter that he's unable to focus on much else.