"Can" is a modal verb and is not conjugated. http://www.elearnenglishlanguage.com/esl/grammar/modalverbs.html

Yes. When "can" is used as a verb, it remains "can" in the third person. "Cans" should be only used when you want to use the noun "can" in the plural form.

because "Can" modal verb! strong verb =)

"Can", as the other members have pointed out, doesn't change. Welcome to intermediate English. There are more rules to learn, so good luck.

El grupo de verbos modales (o verbos auxiliares) no reciben la "s" en la tercera persona (con la excepción de los verbos "to be" y "have" que sí varían). I must, you must, he must. I can, you can, he can. I may, you may, he may. I ought, you ought, he ought. Pero, I am, you are, he is. I have, you have, he has. Espero que eso te ayude.