"Could" also has a hypothetical tone. Can I borrow your pen? = I expect to borrow your pen. Could I borrow your pen? = I don't know if it's OK, it depends on you.

Could is used to ask if you can do something : Could I use your phone ,please? Could we stop by next week ? Used to politely ask somebody to do something for you : Could you babysit for us next Friday ? Used to show that something is or might be possible : I could do it now ,if you like Don't worry, they could have just forgotten to call Used to suggest something : We could write a letter to thre director You could always try his home number Used to show that you're annoyed that somebody didn't do something : They could have let me know they were going to be late Used to emphasize how strongly you want to express your feelings : I'm so fed up I could scream "Could do something" used to say that you need or would like to have something : I could do with a drink Her hair could have done with a wash Ohh I've tired from wrote all of this I hope it'll be useful for you ^_^