Should and Ought to

We can often use should or ought to with little difference in meaning when we talk about OBLIGATION and PROBABILITY.

Obligation

• giving ADVICE or making a RECOMMENDATION:
• 'This soup is too salty!' 'You should / ought to send it back.'
• You'll catch cold if you go out like that. I think you should / ought to take a hat.
or saying what an outside authority recommends (although we prefer should in this case):
• The manual says that the computer should be disconnected (= passive) from the mains before the cover is removed, (rather than ...ought to be disconnected...)
However, we use should (or would), not ought to, when we give advice with I:
• I should leave early tomorrow, if I were you. (or I would leave...; or I'd leave...)
• talking about a RESPONSIBILITY or DUTY:
• People should / ought to be warned (= passive) of the danger of swimming off this beach.
• I should / ought to visit my parents more often.

Probability

• saying that something is PROBABLY TRUE now or will probably be true in the future:
• 'Have we got any string?' 'There should / ought to be some in the kitchen drawer.' (because that's where we always keep it)
• You should / ought to have received the report by now.
• I enjoyed her first novel, so the new one should / ought to be good.
We use should / ought to + have + past participle to talk about an obligation in the past. We often indicate some criticism or regret:
• He should / ought to have asked me before he took my bike. (I'm annoyed)
• We should / ought to have taken a taxi when it rained. (I'm sorry we didn't)
We also use should / ought to + have + past participle to talk about an expectation that something happened, has happened, or will happen:
• If the flight was on time, he should / ought to have arrived in Jakarta early this morning.
• The builders should / ought to have finished by the end of the week.
We can use had better instead of should / ought to, especially in spoken English, to say that we think it would be sensible or advisable to do something. However, we don't use it to talk about the past or to make general comments:
• If you're not well, you should / ought to ask Ann to go instead, (or ...you'd better ask...)
• You should / ought to have caught a later train, (not You had better have...)
• I don't think parents should / ought to give their children sweets, (not ...had better...)
When we make a logical conclusion from some situation or activity, we use must not should or ought to
• You must be mad if you think I'm going to lend you any more money.
• It's the third time she's been skating this week - she must really enjoy it.
We can use (be) supposed to instead of should / ought to to talk about an obligation to do something. It is commonly used in spoken English to express a' less strong obligation:
• I'm supposed to be there at 10.00.
• The work was supposed to start last week.
We use (be) supposed to when we report what many people think is true:
• Eating sweets is supposed to be bad for your teeth, (not ...should be bad for...)