Now if the sites would learn the difference between "cannot" and "can not" (although, I do have to say that many NESs--even those who otherwise write well, use "can not" when they mean "cannot." Grrrrr.
From OED "cannot":
Both the one-word form cannot and the two-word form can not are acceptable, but cannot is more common (in the Oxford English Corpus, three times as common). The two-word form is better only in a construction in which not is part of a set phrase, such as ‘not only … but (also)’: Paul can not only sing well, he also paints brilliantly
There's another way of showing their difference in meaning:
"I cannot take the subway to work today" means exclusively that I am not able to take the subway to work.
"I can not take the subway to work today" can mean either the above (because it was written by someone who writes it as two words) or that it is possible for me not to take the subway to work.
So, it's best to write it as one word for the sake of clarity.