"Can" y "could" se utilizan para expresar permiso, posibilidad o habilidad. Como regla general "can" se utiliza en el presente y "could" en el pasado, pero hay ciertos matices:
a) Posibilidad / habilidad
En el presente "can" y en el pasado "could" o "was / were able".
With the new motorway you can go from Madrid to Barcelona in less than 4 hours
Ten years ago you could go from Madrid to Barcelona in 6 hours
I can play tennis
I could (was able) to play tennis when I was a child
"Could" también se utiliza con un sentido condicional, mientras que en el futuro se emplea "will be able":
If I had money I could buy a new car
In two years I will be able to speak English fluently
b) Permiso
En el presente distinguimos:
- Tener o pedir permiso: "can" tiene un sentido informal y "could" formal.
I can use the company's car for my private trips (informal)
I could use the company's car for my private trips (formal)
Can I borrow your car ? (informal)
Could I borrow your car ? (formal)
- Dar permiso: se utiliza "can"; "could" únicamente se emplea con un sentido condicional.
Tonight you can stay with us
If you don't find any other place, you could stay with
En el pasado se utiliza "could" y en el futuro "can".
c) Ofrecimiento
Sólo se utiliza "can".
Can I help you?
CAN/COULD Requests-Peticiones
Can/Could you give me a larger size, please?
Can/Could I have some stamps, please?
Can/Could you tell me the way to the station, please?
Could I use your telephone? (formal, polite)
Can I wear your green T-shirt today? (informal)
If Susan calls again, can you tell her to phone back later?
If Peter calls again, could you tell him I'm out?
Can/Could you ask her to phone me at work?
Fuente: http://www.aulafacil.com/Ingejerc/Lecciones/Lecc24.htm
http://www.mujerpalabra.net/secundaria/pages/asignaturas/english/verbs_4/
modals4eso.htm
