That's only partly true, Daniella.

The word sequence "can not" is sometimes used for emphasis:

I simply can not believe it!

And sometimes "can not" is required in comparative constructions:

He can not only dance, but he can sing as well!

Note also that the negative form "can't" is more common then "cannot" in speech; indeed it is compulsory when it precedes the subject:

Can't / Cannot / Can not we stay a little longer?


By the way, I gave a link to the Oxford's entry for "can not" in my first answer.


PaulM