CAN- CAN NOT

The auxialiary/modal can has the same form regardless the subject. There is no -s in the 3rd person singular.
The auxialiary/modal can is used with a main verb in its infinitive. It can be used alone in short answers.
There is no contracted form of can in affirmative sentences.

affirmative - long form	affirmative - contracted form	negative - long form	negative - contracted form
I can sing.	---	I cannot sing.	I can't sing.
You can sing.	---	You cannot sing.	You can't sing.
He can sing.	---	He cannot sing.	He can't sing.
She can sing.	---	She cannot sing.	She can't sing.
It can sing.	---	It cannot sing.	It can't sing.
We can sing.	---	We cannot sing.	We can't sing.
You can sing.	---	You cannot sing.	You can't sing.
They can sing.	---	They cannot sing.	They can't sing