In Michael Vince's New First Certificate Language Practice, page 92, exercise 2, sentence number 10, you are given the sentence:

"I thought that you would know better!"

which has to be re-written starting with "You …" and including the word "ought".

The key says "You ought to know better."

Isn't this ambiguous?

Why not "You ought to have known better", since the situation in which the listener made the wrong choice is past, not present?

  • Your answer would have been correct if the question would have been "I thought that you would have known better!"
    – oerkelens
    Mar 3, 2014 at 11:32
  • What, then, is the difference between "I thought that you would know better" and "I thought that you would have known better"? Something to do with the person having learnt a lesson in one case and still not in the other?
    – user58319
    Mar 3, 2014 at 11:39

1 Answer 1


(Ame, non-linguist) I agree with you that you ought to have known better suits most situations well.

I thought that you would have known better: something unexpected happened and the speaker is expressing surprise mixed with a mild reproach.

Considering that you and your companion fight in armor, I thought that you would have known better. - Plato (Euthydemus)

You ought to know better/I thought that you would know better: The event (to my understanding) is still unresolved or there is present danger. For example, a person is trying to pry apart pieces of frozen meat while holding them in his hand.

Stop that! You ought to know better than to do that! (I thought you would know better...)

Or, the person stabs himself (I actually saw this). The other person stares at the fork imbedded in the hand, tines out the other side, and exclaims, You ought to know better than to use a fork that way! while desperately wrapping the bleeding hand-cum-fork. In the Emergency Room, the patient says sheepishly, I really (ought to/should) have known better, to the registrar, the nurse, the doctor, the radiology tech...

  • Yes, I believe I would. Mar 3, 2014 at 13:00
  • So, you would agree with the idea mentioned in my comment, that the choice of a simple or perfect infinitive is to do with a lesson learnt or not, wouldn't you?
    – user58319
    Mar 3, 2014 at 13:11

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