Sergio, Puedes usarlos en cualquier momento con una sola excepción. Puedo cantar = I can sing = I am able to sing. Podía cantar = I could sing = I was able to sing. Podré cantar = ------------------ = I will be able to sing. Podríacantar = I could sing = I would be able to sing. Como puedes ver, el verbo "can" no puede usarse en futuro. Para decir "podré" "podrás"... necesitas usar "to be able to", para cualquier otra forma puedes usar ambos. Espero que eso responda tu pregunta.

Can I just add something to what Mario has said? 1. It isn't possible to use 'can' in the perfect tenses. You have to use 'be able' eg I have been able/ I had been able. 2. 'Can' and 'be able to' are synonyms when the meaning is 'to be capable of'. However, there are many situations where you use 'can' with other meaning: eg Can I have a coffee, please? You can't be serious! It's not possible to use 'be able' in these situations.