Should is the past tense of shall in indirect speech.
- The officer said, ‘The scoundrel shall be given a good beating.’
- The officer said that the scoundrel should be given a good beating.
Duty and obligation
Should can be used with pronouns of all the three persons to talk about duty and obligation.
- We should help the poor and the needy.
- We should not lie.
- We should all work for the common good.
- You should pay the fees in time.
Should can be used in conditional clauses expressing possibilities, suppositions etc.
- If she should come, ask her to wait.
- Should it rain, we will cancel the trip.
Should is often used in main clauses which are preceded or followed by a clause expressing unreal conditions.
- If I were you, I should accept this offer.
- No Sam, I shouldn’t do that, If I were you.
Note that this kind of sentence is often used to give polite advice or gentle admonition.
Should is often used to express possibility or likelihood.
- I should be able to finish this work in time.
- You should be able to beat him.
Should is the only auxiliary verb that can be used after lest.
- Watch and pray lest you should fall into temptation.
Should and shall
Should expresses less possibility than shall.
- I shall be able to meet Peter.
- I should be able to meet Peter.
Here the first sentence expresses a greater possibility of the event – meeting Peter – taking place.