Should is the past tense of shall in indirect speech.
- Direct: He said, ‘the scoundrel shall be trashed.’
- Indirect: He said that the scoundrel should be trashed.
Should has the following uses:
To express duty or obligation
Should is used with subjects of all the three persons to express duty or obligation.
- We should be kind and honest.
- We should have helped him.
- You should tell him the truth.
- They should pay the fees in time.
In conditional clauses
Should can be used in conditional clauses expressing possibilities, suppositions etc.
- If he should come, ask him to wait.
- Should it rain, there will be no match today.
The clause ‘if he should come’ indicates less likelihood of ‘his coming’ than ‘if he comes’. It means something like this: ‘There is not much likelihood of his coming. But if does turn up ask him to wait.’
Should after lest
Should is the only auxiliary verb that may be used after lest.
- Watch and pray, lest you should fall into temptation.
Should like to
Should is used in the expression ‘should like to’ which is a polite form of making a statement.
- I should just like to mention that we have only two days to make a decision.
To express possibility or likelihood
- I should be able to meet them.
- We should be able to finish the work in time.
Should expresses less possibility than shall.
- I shall be meeting him tomorrow.
- I should be meeting him tomorrow.
Here should adds a coloring of doubt to the second statement whereas the first sentence expresses a greater possibility of the event taking place.