Suppose that in a conversation, our conversational partner just said something completely new to us, and we want to express that we didn't know (or haven't known) about it before. For example,

A: Some dictionaries will tell you whether a noun is countable or not.

B1: Oh, I didn't know that.
B2: Oh, I haven't known that.
B3: Oh, I didn't know that until you mentioned it.
B4: Oh, I haven't known that until you mentioned it.

Which responses are possible? (I think all are possible.)
Which one is the most likely response by a native speaker?

  • Note: Any speakers are welcome to answer. In case that the answer is dialect-specific, please say so. Thank you in advance. Commented Apr 22, 2014 at 23:19
  • 1
    Only B1 and B3 are correct, though I can't come up with a thorough explanation as to why at the moment. It's not dialect-specific, haven't just doesn't work here. :) +1! Oh, as for most likely.... I'd say B1, just because usually shorter is most likely in most cases. But there's no reason B3 wouldn't work; it doesn't sound unnatural or anything.
    – WendiKidd
    Commented Apr 22, 2014 at 23:27
  • 2
    (By the way: you should say in *the case that* or if; in ∅ case is a different idiom meaning "to anticipate [a possible future problem]".) Commented Apr 22, 2014 at 23:48
  • 1
    I had never heard of that before.
    – user3169
    Commented Apr 23, 2014 at 0:06
  • 1
    I think "Oh, I hadn't known that." would work wouldn't it?
    – user13267
    Commented Apr 23, 2014 at 2:51

1 Answer 1


You cannot use I haven't known that here. The present perfect describes a present state which arises out of a prior eventuality, and you are implicitly saying that your present state is that you do know that—which is not a state that can arise out of your previous ignorance. (It can emerge from your previous ignorance, but it can only arise out of learning it.)

You may use I didn't know that, with or without until you mentioned it. This states that in the past you were in a state of ignorance, a state which ended at the point when your addressee mentioned 'it'. Until marks the end of a state, as explained here.

You are not obliged to include anything like until you mentioned it in order to make it clear that now you do know that—the discourse situation takes care of that—it serves only to make clear that it was your addressee's statement which dispelled your ignorance, not some other past event.

You may also use I hadn't known that, again with or without until you mentioned it. The past perfect does not necessarily describe a state arising out of a prior eventuality, because the past tense-domain does not have the contrast between simple past and present perfect which exists in the present domain—the past perfect serves for both. Here the past perfect acts as a “past-in-past”, analogous to the present-tense-domain simple past, so it describes a prior state of ignorance which ended at your past reference time, the time explicitly named in the until clause.

However, as you know, FumbleFingers' Perfect Truism instructs you that if you do not need a past perfect you should not use it. In this case there is no evident reason why a past perfect should be needed; consequently the best choice here is the simple past:

I didn't know that (until you mentioned it).


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