“Cannot” and “can not” might seem like they mean the same thing, but you use them in different ways.
“Cannot” is usually the word you want. It means “unable to” or “unwilling to” do something.
- I cannot come to rehearsal tonight.
- Mom said I cannot have the car tomorrow.
“Can not” is occasionally used as an alternative to the one word “cannot,” but it shows up most often when the word “not” is just part of something that comes right after “can.” For example, use “can not,” (two words) when “not” is part of a “not only … but also” construction.
- You can not only be in the play, but also choose your understudy.
- You can not only have the car, but you can also get the car washed on your way home.
“Can’t,” the contraction for “cannot,” is just a more informal replacement for the one-word form of “cannot.”
- Mom said I can’t have the car tomorrow.
Don’t use “can’t” where you would use the two-word version.
Also, when I was a professor, I saw my students write “cant”—without an apostrophe—with surprising frequency when they meant to write “can’t,” the contraction for “cannot.” I don’t know if they were using voice recognition software and it was getting it wrong or what. It was weird. I probably should have asked, but we always had bigger fish to fry.
“Cant,” without the apostrophe, is a real word, but it’s uncommon. It can refer to jargon or a private language such as one spoken by gangsters or other underworld characters, as in The cant of mobsters obscures the violence of their exploits. Cant can also refer to talking like you are begging or whining. It has a lot of meanings, but for the most part, you’re not going to use most of them.
So to sum up, “cannot” is the word you want most of the time. Only use the two-word version, “can not,” when “can” and “not” just happen to fall next to each other in a sentence. The contraction “can’t” is fine anywhere you’re comfortable using contractions, which are a little more informal than writing out all the words, and always remember to put the apostrophe in “can’t” when you are writing the contraction.
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Mignon Fogarty is Grammar Girl and the founder of Quick and Dirty Tips. Check out her New York Times best-seller “Grammar Girl’s Quick and Dirty Tips for Better Writing.”