Cannot means to be unable to do something or to lack the ability, permission, or possibility to do it. It's the opposite of can. When you say you cannot swim, you mean you don't know how. When you cannot stay up past your bedtime, the rules don't allow it. When a plant cannot survive without water, it's impossible for it to live.

People often say can't in everyday speech. You might write “I cannot attend the party” in a formal note, but say “I can't come” to a friend.

Understanding what you cannot do is just as important as knowing what you can do. Trying to do something you truly cannot do can be dangerous and shows a lack of understanding of reality. But sometimes what seems impossible turns out to be achievable with practice and persistence. A student might think they cannot solve a difficult math problem, but with effort and the right help, they discover they can.