Do you use the word "cannot" ?

Is it just me? I have trouble writing it, although it is preferred to can not, for example here. There are no words didnot or maynot, so why should there be cannot? Sometimes, cannot does not work. If someone asks me if I can fast for a day, I could say that “I can not eat for a day,” which means something entirely different from “I cannot eat for a day. ”