Use of should and would

1.  Should and would are used as past tense of
     shall and will; as,
     I said that I would go. (Indirect of I said, "I will
     go")
     I promised that you should have a reward.
2.  Should is used
     i)  to express duty or obligation:
          We should obey our parents.
          You should keep your promise.
     ii) to express a supposition in a conditional
          clause:
          If he should come, ask him to wait.
          If it should rain, we shall cancel the party.
     Note: Should is as less forceful than must.
3.  Would is used to express a polite request in a
     question form:
     Would you lend me your book?
     Would you mind coming with me?
     Note: 'Would you' is more polite than 'will
                 you' though both imply the same thing.