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)