Advisability

Advisability: Should, Ought to, Had better.

You should go and see a doctor	Should: This is a good idea
You ought to go and see the doctor.	Ought to: This is important
You had better go and see the doctor or your child will get contaminated.	Had better: This is a warning
Negative forms: Should not Ought not Had better not	Had better is followed by the simple form. Had better is stronger than should/ought to, and implies a warning or possible bad consequences.

Necessity	Advisability
Have to Have got to must	Should Ought to Had better

The past form of should

I had a test this morning. I didn’t do well on the test because i didn’t study for it last night. I should have studied last night.

Should have + past participle