Can - Cannot

Can is a modal verb. We use it to express ability.

Can is the same for all subjects. We don't add an 'S' in the third person (like other verbs)

Affirmative

The verb that comes after Can is in the infinitive without to. For example:

SUBJECT + CAN + MAIN VERB (INF) + COMPLEMENT

• John can climb a pile of chairs. (= He has the ability to climb a pile of chairs.)
• Carol can skate on ice.
• I can ride a bike 
• You can swim.

Negative

To form the negative we add "not" after can: cannot. We use cannot to express lack of ability to do something.

We can also contract the negative to form can't. (can't = cannot). For example

SUBJECT + CANNOT (CAN'T) + MAIN VERB (INF) + COMPLEMENT

• They cannot sing very well. (= They don't have the ability to sing very well.)
• Tom can't dance well. 

Questions

To from the question we change the position of the subject and the auxiliary verb.

CAN + SUBJECT + MAIN VERB (INF) + COMPLEMENT?

The main verb is still in the infinitive without to.

• Can John climb a pile of chairs ? 
• Can Carol skate on ice? 
• Can they sing well? 

To answer the questions we use:

YES/NO + SUBJECT + CAN / CAN'T

For example

Can you sing? Yes, I can - No, I can't 

Watch this video to have more examples: https://www.youtube.com/watch?v=zFdazgM7r-8