should

modal verb (3rd singular should)

1》 used to indicate obligation, duty, or correctness.

    ↘used to give or ask advice or suggestions.

2》 used to indicate what is probable.

3》 formal expressing the conditional mood.

    ↘(in the first person) indicating the consequence of an imagined event.

    ↘referring to a possible event or situation.

4》 used in a clause with ‘that’ after a main clause describing feelings.

5》 used in a clause with ‘that’ expressing purpose.

6》 (in the first person) expressing a polite request or acceptance.

7》 (in the first person) expressing a conjecture or hope.

Origin

OE sceolde: past of shall.

Usage

As with shall and will, there are traditional rules about the use of should and would. These state that should is used with I and we (I said I should be late), while would is used with you, he, she, it, and they (you didn't say you would be late). However, these rules are no longer strictly followed and would is normally used instead of should in reported speech and conditional clauses: I said I would be late. In spoken and informal contexts the issue rarely arises, since the distinction is obscured by the use of the contracted forms I'd, we'd, etc.

For a discussion on the use of should of instead of should have, see usage at have.