When should I use CAN and when should I use CANNOT?

We use CAN in positive sentences. e.g. I can see a chair.
We use CANNOT in negative sentences. e.g. I cannot see a chair.
But remember, either we use CAN or CANNOT, the following verb must be a bare infinitive. Also, CANNOT is one word only, not two words. The short form of CANNOT is CAN'T. The past tense of these two auxiliary verbs are COULD and COULD NOT (COULDN'T). We can also use COULD in asking requests. This is the way for us to ask politely.

There is no such term as 'can not'.

The negative of 'can' is always 'cannot'.

Or you can use the short form-----can't