"Ought" is used to express some moral obligation or probability. "Should" is used in the same way. Some people think it has less of a moral flavor, but this is debatable. "Should" also has other uses: Conditional - "If he were to come, I should be very suprised" = I will be very surprised. Its use is considered more polite and less direct than "would". "I should think he would have stopped by now." - Same usage. In the above examples "ought" cannot be substituted for "should". "The train should be here by now" = "The train ought to be here by now" In the case of obligation or probability, 'ought' can be put in place of "should" and visa versa. Also, the English tend to use 'ought' more than the Americans.

I don't know ... I use them interchangeably. I suppose I should know the answer. I suppose I ought to know the answer. Hmm. They both sound perfect. I should go now. I ought to go now. Hmm. They both still work. I'll just go.

Semantically, there is no real difference between "ought to" and "should". these two can be used interchangeably. But still, people use "should" much more often than "ought to". For most English speakers, these words carry a nearly identical sense of obligation. If you are really eager to know.. mhm.. sure there are some minor connotative differences ( which you really do not need to know, because I am pretty sure 90% of Americans must be dizzy of what I am about to say) The exact meaning of SHOULD is " Give the suggestion on the right thing for someone to do". However, for OUGHT TO, it means "Indicate what someone should do". The only difference is that OUGHT TO indicates the last&only option someone can choose and SHOULD shows the best option someone can take. For example. It must be raining tomorrow. --You OUGHT TO take an umbrella with you. (It is your only option to not get all wet!) It might be raining tomorrow-- You SHOULD take an umbrella with you. (it is the best choice for you to not get all wet! )

I understand now. Thanks a lot for your help¡ I'm pleased have friends as you.

You ought not be concerned about their difference. I agree with Steve and some others that they can be used interchangeably.