"I didn't know that" vs "I don't know that"

[Nessie]

Hi,

Please imagine a context like this:

A: Lake Baikal is the largest lake in the world.
B: So? I didn't know that/ I don't know that

=> I've just watched an American film in which the actor said "I didn't know that". I think it is correct because B hadn't known that fact until A told him/her. But after A had said so, B knew it and so B's original sentence should be in past tense. I just want to know it the usage is common in American English, and is it the same in British English?

Many thanks in advance.
Nessie
_________________
... something we never have again, I know... I guess I really really know..

Sorry seems to be the hardest word...

[Rajin]

In American slang it's sometimes common to use "don't" instead of "didn't" depending on the person and someone might say "I don't know that" as if to say "Of course I don't know that knowledge"

Another example is "It don't even matter." instead of "it doesn't even matter." from Rush Hour (Chris Tucker)

"I don't know that" can also be used if the person doesn't trust the first person completely. For example: A->"There's $1000 in your bank." ...B->"What? I don't know that (for certain)."

[MrPedantic]

In BrE, I would expect "I didn't know that".

If I were to say "X is the case", and someone were to reply "I don't know that", I would assume the speaker meant "I can't be sure you're right", as in Rajin's last example.

All the best,

MrP

[Molly]

--- Edited. Wrong thread.

[Yankee]

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Why just edit such a post, Molly?
Didn't you know that it is possible to delete your own post when it is still the last one in the thread?
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[Lost_Soul]

[Yankee]

[Alan]

Hi,

I really can't see the variations of or differences between 'I don't know' and 'I didn't know' having all these subtle meanings. Don't they simply mean what they say? The tone of voice with which they are spoken says it all, doesnt it? But ultimately that applies to everything!

Alan
_________________
English as a Second Language
You can read my ESL story In Order

[Yankee]

Hi Alan

I assume you'd agree that the following two sentences would mean different things (as a response to you last post):

- I don't know that I'd agree with you, Alan.
- I didn't know that I'd agree with you, Alan.
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[Molly]

[Yankee]

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Thanks for pointing out the typo, "Molly". Do you suppose Alan will want me to correct it?
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[Alan]

Hi,

I actually said:

[Yankee]

[Nessie]

[Alan]

Hi,

I honestly believe that quibbling about the way someone says it or doesn't say it with regard to whether it's 'I don't know' or 'I didn't know' is a complete waste of time. As I have already said and I promise I'll not say it again - Both phrases mean exactly what they say.

Alan
_________________
English as a Second Language
You can read my ESL story Womens' Day