Can is a modal verb and so always has the same form:
After can we use the base form of the verb.
I can swim.
She can speak Spanish.
The negative form of can is can’t.
We use can to talk about ability and permission.
They can drive.
She can’t use a computer.
He can’t go to the party.
They can park there.
As with all modal verbs, to make a question with can we invert can and the subject:
You can swim. (affirmation)
Can you swim? (question)
He can speak German. (affirmation)
Can he speak German? (question)