Advisability: Should, Ought to, Had better

Examples:

(a) You should study harder.
(a) You ought to study harder.

(b) Drivers should obey the speed limit.
(b) Drivers ought to obey the speed limit.

Should and ought to have the same meaning: they express advisability. The meaning ranges in strength from a suggestion ("This is a good idea.") to a statement about responsibility or duty ("This is a very important thing to do.").
In (a): "This is a good idea. This is my advice."
In (b): "This is an important responsibility."

(c) You shouldn't leave your keys in the car.

Negative contraction: shouldn't

(d) I ought to ("otta") study tonight, but I think I'll watch TV instead.

Ought to is sometimes pronounced "otta" in informal speaking.

(e) The gas tank is almost empty. We had better stop at the next service station.
(f) You had better take care of that cut on your hand soon, or it will get infected.

In meaning, had better is close to should/ought to, but had better is usually stronger. Often had better implies a warning or a threat of possible bad consequences.
In (e): If we don't stop at a service station, there will be a bad result. We will run out of gas.

Notes: Had better has a present or future meaning. It is followed by the simple form of a verb. It is more common in speaking than writing.

(g) You'd better take care of it.
(h) You better take care of it.
(i) You'd better not be late.

Contraction: 'd better, as in (g).
Sometimes in speaking, had is dropped, as in (h).
Negative form: had better + not.

Ought to is not commonly used in the negative. If it is used in the negative, the to is often dropped: You oughtn't (to) leave your keys in the car.


Cheers!
Naeem