I haven't known that vs. I haven't known that before

[shootingstar]

I have learned the sentence I haven't known that is grammatically incorrect (see the link below). What about the second sentence I haven't known that before? Isn't that the same as in the sentence We haven't met before?

I feel as if I have opened a can of worms vs. I feel as if I had opened a can of worms

I've learned from the annexed thread that I feel as if I had opened a can of worms is wrong. I don't know the reason, though. Please tell me why it is grammatically incorrect to write had. In my opinion both versions are possible. Thank you...

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[jutfrank]

Let's clarify something first of all.

I have learned the sentence I haven't known that is grammatically incorrect (see the link below).

No, out of context it is not ungrammatical, and nobody has said it is. It's only incorrect if it's not what you mean.

What about the second sentence I haven't known that before? Isn't that the same as in the sentence We haven't met before?

It has the same grammatical form, yes. They're both present perfect.

You obviously have typical German difficulties in understanding the use of the present perfect. You have to learn to distinguish when you're talking about the present and when you're talking about the past. In your response, you're talking about a past point in time (a time before you told me), so the past tense is the right one.

 

[shootingstar]

Thank you.
To cut a long story short, can I say I haven't known this difference in meaning before - being grammatically correct and corresponding to what is meant (i.e. the tense)?

 

[emsr2d2]

To cut a long story short, can I say I haven't known this difference in meaning before, being and is it grammatically correct and corresponding when referring to what is meant I'm talking about (i.e. the tense)?

I can't think of any context in which "I haven't known this difference in meaning before" is usable. I'd accept that "I haven't known [about] this difference in meaning for very long" is possible, in which the end of the sentence refers to a duration ("for very long").