Ought to is used in two ways.
1. Logical Necessity
One way of using ought to is to say that we think something is probable, because it seems logical or normal. Look at these sentences:
1. Your parents ought to be here soon; they left home an hour ago.
2. A: We will spend the holiday on the beach.
B: That ought to allow you some time to tell your wife about your problems.
Apart from this use, ought to can be used to impose and ask about obligation: to advise people (including oneself) to do things or to tell people that they have a duty to do something or to enquire about one’s duty.
Consider the sentences below:
3. I really ought to go and see my sick aunt.
4. What time ought I to do this work?
5. She ought to be more careful.
6. Women ought not to talk to men in that manner.
In both these meanings, ought to have a very similar meaning to should.
Ought to and should are like weaker forms of must. Compare:
7. That must be my father (= I’m certain it’s my father.)
8. That ought to be my mother (=There’s a reason to think it’s my daughter.)
9. You must tell the police (= an order, or very strong advice)
10. You ought to tell the police (= advice)
As the example above shows ought to expresses ‘probability’, and can be regarded as a weaker equivalent of must.
Note that, ought to expresses an obligation, which may not be fulfilled. Look at the sentences below:
11. I really ought to go an condole with the Daniels
(‘but I probably won’t have time)
Ought to can be followed by a perfect infinitive to talk about the past:
12. You ought to have known we were coming.
13. The bomb ought to have exploded now.
Ought to is often felt to be awkward in the questions involving inversion, and should, is preferred; so that, instead of:
14. Ought we to go at this time?
Some speakers prefer
15. Should we go at this time?
Note, however, that questions and negatives are not (normally) made with do:
16. Ought you to smoke at all?
17. You oughtn’t to smoke at all.

Credit: Basic English (A Trouble-shooting Approach): Adeleke A. Fakoya (1999)