In most cases SHOULD and OUGHT TO are used interchangeably today. Both SHOULD and OUGHT TO are used to express advice, obligation or duty. However, there is a slight difference in meaning. The “SHOULD is used when we want to express our subjective opinion.The difference is that OUGHT TO is stronger in meaning. For instance, We should respect each other and help each other. Every citizen ought to perform military service.

Hi Danyel, For the sentence "You ought to have known it", notice the phrase "have known". In this context, you are telling the other party that he or she should have obtained some knowledge of something as a reflection. For example: "If they had known of the danger, they would not have entered the cave." In this case, they had entered the cave. Probably they got into some danger. After they escaped, they reflected about the entire incident and felt that if they knew of the danger BEFORE they were about to enter the cave, they would not do so. I hope this helps. Cheers, Lance

Greetings, There is difference between 'should' and 'ought to'. When we talk about giving advice, both can be used.But 'ought to' is much stronger and can sound bit harsh or rude. Ex-1. You should drive more slowly in this freezing weather. (note: if you say ‘You ought to drive more slowly When we talk about expectation, 'ought to ' is less common. Ex-1. My father should be here by now – he usually arrives at this time. (note: if I say ‘My father ought to be here by now…’ it sounds like I may be a bit annoyed that he is not here yet. ) Hope it helps. Regards, Deepika Mantra