Historically it was the past
form of the verb "Shall" and both of the forms expressed
obligation. But in present day English they have developed different
meanings and are treated as 2 different verbs.
"Should" followed by
the non perfect infinitive may be used with the reference to the present
and future and is not changed in the reported speech. For example:
You should be more
I told him that he should
be more careful.
This modal verb is used only
in two meanings.
- Moral obligation or duty in all
kinds of sentences/about the present/.
All the students should hand in
the papers today.
- Probability only with the
reference to the present of future
The guests should not come
- In the negative sentences
"Should" denotes negative advice.
He should not be so impatient!
- In the set expression "Why
Why should I go there?
- "Should" + Perfect
Infinitive indicates that something desirable has not been
done. It denotes criticism only in the affirmative sentences.
You should have put more sugar
in my coffee. It's not sweet