Cannot or can’t?

I was recently asked if it is grammatically better to use can’t instead of cannot.

Actually, both words are grammatically correct and context is the best basis for choosing which word to use.

When writing something formal, the word cannot is more appropriate; when writing soemthing informal and casual, you tend to use words as you would speak them, so can’t is quite appropriate and used more often.

There is nothing wrong with using either word in any context, but matching the word to the context helps the flow and style of your writing.