OUGHT TO

*Ought to has the same meaning as should.
*It's used when we giving advice, incluing to ourselves, to say what we think is the best or the right thing to do.
My friend doesn't look very well. She ought to see a doctor.

What's the difference between ought to and must?
*Must is used to express strong obligation.
I must go to the hospital now, my mother had an acident.
*Ought to is used to express strong probility.
I ought to call John.

The negative form of ought to is formed by placing not between ought and to:
Mary ought to be driving.
Mary ought not to be driving / or / Mary oughtn't to be driving

The negative form ought to is normally used in British English. The form shouldn't is used in Americam English.
You oughtn't to call Beth for a party.(rare)
You shouldn't call Beth for a party.(common)

The interrogative form of ought to is formed by placing the subject between ought and to:
He ought to learn drive a car.
Ought he to learn drive a car?
It's also very rarely used.
What time do you think I ought to arrive at Beth's party?
What time do you think should I arrive at Beth's party?

In the tag questions, the particle to is not used. Look:
My friend oughtn't to be so rude, ought he?
My friend to be more polite, oughtn't he?
To form the affirmative tag: ought + pronoun
To form the negative tag: oughtn't + pronoun