OUGHT TO and SHOULD : They are often interchangeable but only in the sense of obligation.  Similarly ‘ Ought to have -ed ‘ can be replaced by ‘ Should have  -ed ‘. The moral force of ‘Ought to ‘ is more than ‘ Should ‘ , but less than ‘ Must ‘.  The moral force concerning the above can be arranged in the following ascending order.

SHOULD——— OUGHT TO———–MUST.

• You ought to help him.  OR -You should help him. (i.e you are obliged to help him)
• You ought to have gone there.  OR – You should have gone there( i.e you were obliged or supposed to go there but you didn’t do)

The negatives are :

Ought to                           – Ought not to

Ought to have———ed   – Ought not to have—–ed

Examples:

• You oughtn’t  to tell him about the incident. OR – You shouldn’t tell him about the incident.  ( i.e something that you should not tell him)
• You oughtn’t to have told him about the incident. OR – You should not have told him about the incident.( i.e you told him something which you were not supposed to)