Andrew, a fellow English teacher, writes:
One of my ESL students asked me a question that I could not find an answer to:
Why is "cannot" written as one word?
Just because! I know. But perhaps there is a better answer? I defer to your experience. Admittedly, it is a question that has haunted me for some time.
I've often wondered the same thing, so this question got me moving—toward my dictionary. There I find that "cannot" is "the negative of the auxiliary verb can: written can not for emphasis."
So I speculate that at some time in the past, people stopped writing "can not" as two words, and the single word became the general usage. Emphasis would fall on the first syllable, as it usually does in two-syllable English words: I CANnot do this.
But I can see how breaking it back into two words would shift the emphasis to the negative: I can NOT do this.