These two spellings are largely interchangeable, but by far the most common is “cannot”; and you should probably use it except when you want to be emphatic: “No, you can not wash the dog in the Maytag.”
This is the tenth year of the Common Errors in English Usage calendar. To celebrate, we are bringing back some of our favorite interesting, funny, but sometimes merely silly entries through the years before going on hiatus in 2016.
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