The Forms, Meanings and Use of the Modal Verb CAN (COULD).

Can and could are modal auxiliary verb, like for example must, may and should. Like other modal verbs, 'can' has no �s on the third person singular (he can, not he cans); questions and negatives are made without do (Can I?, Could I? not Do I can?, Did I could?); these verbs are followed by the infinitive without to (e. g.: I can do it.). 'Can' and 'could' have no infinitive or participles (to can, canning do not exist).

Patterns. Read and memorize!

Affirmative	Interrogative	Negative
I can swim.	Can I swim?	I cannot swim.
He can swim.	Can he swim?	He cannot swim.
She can swim.	Can she swim?	She cannot swim.
It can swim.	Can it swim?	It cannot swim.
We can swim.	Can we swim?	We cannot swim.
You can swim.	Can you swim?	You cannot swim.
They can swim.	Can they swim?	They cannot swim.

Note 1: - the modal verb 'can' has only two forms � can in the present and could in the past. The missing tense is supplied by the future of to be able.
E. g. : I shall be able to prepare for my exams.

Note 2: - There are contracted negative form can�t, couldn�t.
E. g.: I can�t understand.

Be careful about the pronunciation of 'can�t': in British English it has a quite different vowel from 'can'.
Can [k?nt]; can�t [k??nt] (GB), [k?nt] (US).

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