Use of Can and Could

Can and could
1.  Can is used to express the ability or capacity to do something; as,
      He can (= has the capacity to) cross the river.
      I can (= am able to) drive a car.
2.  Can or may is used to seek or grant
      permission,  can in an informal manner but
      may in a more formal and polite manner; as,
      Can I go? (quite casual and informal)
      yes, you can go home now.
      May I leave now? (more polite and formal).
      You may go home after finishing the work.
USAGE: Earlier it was uncommon to use 'can' to
               give or seek permission. Now can is
               common instead of may for asking or
               giving permission, so that children say
               'Can I come in?' or 'Can I leave the
               table?'
3.  Can or may is used to express a possibility;
     can shows a theoretical possibility but may
     shows a factual and positive possibility; as,
            Anyone can make a mistake.
            My uncle can visit us any day.
            The sky is overcast with clouds. It may
            rain.
            I am feeling well today. I may go to my
            office.
4.  May is used to express a personal wish; as,
            May you live long! May God bless you!