"Can" is a weird verb. It works in the present tense (I can, you can, they can) and the past (I could, you could ...). But in the future tense it doesn't work -- I will can? (Unless you're talking about putting things in aluminum containers, which is a totally different verb.) There's no infinitive either -- to can. Or a gerund -- I am canning. Aside from those two specific tenses where it works, you have to fall back on "to be able." Today I can, yesterday I could, tomorrow I will be able. "May" is the same way, although it doesn't even have a past tense. Weird.