Use of Could and Might

1.  Could and might are used as past tense of can
     and may; as,
          He said to me that I might/ could go.
          (Permission) permission
          I could swim across the river when I was
          young. (Ability)
          We thought he might come. (possibility)
2.  In modern usage could and might are used as
     less positive version of can and may; as,
          Might/ could I borrow your scooter? (Less
          positive but more polite than 'May/ Can I
          borrow?')