Ought is a little different from the other modal verbs because it needs to in front of the infinitive, e.g. You ought to tell him. It has several functions but its most common uses seem to be to express some kind of obligation or to make a deduction, as in the following sentences:
It can be used to refer to past obligations or deductions as in the following:
In all of these sentences, ought to to could be replaced by should with no change of meaning. And in fact, should is more common than ought.
Which of the following sentences is correct?
The ticked sentence is correct. The others are not possible. (It is interesting that the modal to need allows two forms. Both I don't need to go and I need not go are correct.)