Is 知らないでしまった a thing?

Question

I was trying to tell a language-exchange friend that I didn't know something with the somewhat express connotation that I should have (known).

The first thing that came to mind was しまう, which usually expresses unintended action that is done. However, this immediately became a problem, as I did not know how to use しまう with the negative て form. According to this topic, ないでしまう is acceptable.

知らないでしまう

Yet, this usage seems to be rather rare.

In short, if I were to say, "Oh, shoot, I didn't know that," what is the most natural and/or common way to express this? Is 知らなかった alone enough?

よろしくお願いします。

Answer 1

As you guessed at the end of your question, 知らなかった is actually enough to say, in order to tell the person you're speaking to "I didn't know that."

知らないでしまう sounds very ungrammatical to me, and 知らなくてしまった　doesn't sound much better. The first sounds like you are asking the other person not to know something, almost. The second sounds like you accidentally succeeded in not knowing something.

to express "self reproach" or in other words, the "shoot" feeling, you don't have to change the word 知らなかった, just intone it with emphasis. If you want to truly emphasize that you feel chagrined, you could try elongating the last syllable, but if you've never heard the way that sounds, you might end up sounding very odd.

I might try to express this feeling with something like:

あ、そうか、そうか、知らなかった！

if you really want to say to the person that you "should have known that" you can actually say so with:

知らなかった！知るべきだった(のにね)。

or

知らなかった！知ってたはず(なのに)。