今日は母の誕生日だ!気がついていなかった。

So the hint is past, negative, standard.
Why can’t I use standard instead of progressive?
きがづかなかった
[i didn’t notice]
If my answer is wrong for some other reason, that’s OK too.

Perhaps it was marked wrong because you wrote 「づかなかった」(with づ) rather than 「つかなかった」? Just a guess.

BunPro usually has dots at the bottom of a grammar question after an answer has been inputted, showing the different options possible. Did you check that? Or were there no other options?

1 Like

That was it, thanks.

1 Like

Cool! Glad I was able to help.

Yeah, as you can see, when it comes to that phrase, you can either write 「気がつく」 or 「気づく」, but not 「気がづく」.

Take care!

1 Like