CAN

The verb " can" is a modal verb that means something is possible (possibility) or that someone can do something (skill or ability).
This verb is always used together with a main verb in its infinitive form.
Modal verbs in the third person singular does not add the "s" added to other verbs in english.

AFIRMATIVE FORM:

Estructure:

Subject + modal verb + verb infinitive wiythout to.

Example:

I can play tennis.
He can open the window.

NEGATIVE FORM:

Subject + modal verb + not ( can´t) + verb infinitive.


Example:

I can't go to the party.
We can not smoke in the university.


INTERROGATIVE FORM:

Can + subject + verb infinitive without to

Example:

Can you come with us?.
Can she finish the task of English?.

http://www.englishpage.com/modals/can.html