One of the meanings of the modal auxiliary
should is advisability:
--You should leave right now.
--They should fill out this form. You
can also express the same idea using either
ought to or had better:
--You ought to leave right now.
(You had better leave....)
--They ought to fill out this form.
(They had better fill out....)
Should and ought to both mean that something
is a good idea. Had better is usually
stronger, implying a warning of bad consequences.
The negative of should is shouldn't. Ought
to is not usually used in the negative.
Often you will hear people pronounce ought
to as otta.