There are social conventions in English (and probably in other languages) in which we don't say literally what we mean. We leave it to the listener to figure out the implications of the words we use instead of saying what we really mean. The purpose of such conventions is to avoid always ordering people around!
Two concepts which are frequently used this way are in fact opposites of one another.
Asking someone if he has the ability to do something when it is obvious he does is a way of saying "Do this!"
Asking someone if he has a need or obligation to do something when it is obvious that he does not is a way of saying "Don't do this!"
Can you pass the salt? Could you pass the salt? Can you reach the salt? Could you reach the salt? Can you step aside? Could you lend me that pen? Could you be more considerate of your brother? (= Pass the salt, (please), several ways; Step aside; Lend me that pen; Be more considerate ...)
Must you spill the salt all over the table? Must you do that? Do you have to track your muddy boots into the kitchen? Must you always have everything your own way? Did you have to tell everyone about that embarrassing episode? Do you really need to tie up the phone for an entire hour? (=Don't spill the salt ...; Don't do that!; Don't track your boots ...; Don't always insist on having everything your way!; You should not have told everyone ...; Don't tie up the phone ...)
![Geeked [8-|]](/emoticons/emotion-15.gif)
CJ