moral obligation; duty; advice
*You should not eat so many sweets.
*You ought to make an effort
2. criticism of a past action
*You should have come straight to me.
*You ought to have tried.
3.strong probability, supposition,
near certainty (about the present or future only)
*Potatoes should/ought to grow well here.
4.SHOULD is used in set expressions:
Why should (not) smb do sth?
Why should (not) smb hane done sth?
Corresponding to Russian с какой стати? : *Why should he interfere?
SHOULD/OUGHT TO followed by the indefinite or continuous infinitive expresses advice, if followed by the perfect infinitive, SHOULD/OUGHT TO expresses criticism of a past action: *You should not say much things. *You should not have said such things. (=an undesirable action was done)
SHOULD and OUGHT TO on the meaning of near certainty are usually found in affirmative sentences and in combination with the indefinite infinitive. They express near certainty about the action present or future only, and are seldom used in this meaning, which is normally rendered by MUST or PROBABLY (in negative sentences referring to the future):
Present: *She knows all about it.
*She must know all about it.
*She should/ought to know all about it.
Future: *She will grow up pretty.
*She should/ought to grow up pretty.
*Probably she will grow up pretty.
Past: *It rained in the night.
*It must have rained in the night.
3. OUGHT TO is used in some set patterns to express reproach: *You ought to know better.
4. SHOULD for emotional colouring («emotional» SHOULD): *Why should I help them?
MUST, SHOULD AND OUGHT TO COMPARED
All the three verbs serve to express obligation. MUST, however sounds more forceful, peremptory. Both SHOULD and OUGHT express moral obligation, advisability, desirability. They are very much alike in meaning and often interchangeable. OUGHT is however more emphatic and expresses what is generally accepted, stresses the meaning of duty, moral obligation. SHOULD presents an individual opinion and is more common in instructions.