Hello Ruaner, All of these verbs are a family called modal verbs and they have different functions in different circumstances. For example they may be modals of obligation, advice, deduction and others. It is best to learn them in their individual situations. There are a few concrete rules - 1 modal verbs never change for people, modal verbs are always followed by a verb in infinitive form (no 'to') Hope this helps Bob

can, could - be able to I can speak spanish. - I am able to speak spanish I could not speak spanish. - I was not able to speak spanish But you can also use it like this I could speak spanish, but I will not. ------- This is not really past tense. You can even say 'Tomorrow, I could speak spanish, but I won't.' ------- it is just that the decision not to, has already been taken...

http://www.learn-english-today.com/lessons/lesson_contents/verbs/modals.html