→ This expression is used to emphasize the degree of a particular situation or state of affairs. It can only be used in declarative sentences and can be used with both verbs and adjectives.
→ It is an ironical expression of strong affirmation by using '-(으)ㄴ지/는지/었는지 몰라요' It means the thing the speaker has experienced or knows about is emphasizing the nuances of 'so much so that.' '얼마나 다고요./(느)ㄴ다고요.' is a variant form.
Example:
과거 A/V 얼마니 -았/었는지 모르다 (past tense)
비싸다 -----------> 얼마나 비쌌는지 모르다
싸가 -----------> 얼마나 쌌는지 모르다
먹다 -----------> 얼마나 먹었는지 모르다
가다 -----------> 얼마나 갔는지 모르다
현재 얼마나- A-(으)ㄴ지 모르다 V -는지 모르다 (present tense)
예쁘다(A)-----------> 얼마나 예쁜지 모르다
작다(A) -----------> 얼마나 작은지 모르다
먹다( V ) -----------> 얼마나 먹는지 모르다
가다( V ) -----------> 얼마나 가는지 모르다
Example
Conversation 1
가: 도쿄 여행은 어땠어요? 재미있었어요?
나: 네, 그렇지만 도쿄의 물가가 얼마나 비싼지 몰라요. 물가가 비싸니까 쇼핑을 마음대로 못 해서 아쉬워요.
A: How was your trip to Tokyo? Was it interesting?
B: Yes, but I can't tell you how expensive things were there. I can't shop as much as I want because the prices are high.
Conversation 2
가: 왜 그렇게 놀란 표정이에요?
나: 책을 읽고 있는데 가자기 문이 열려서 얼마나 놀랐는지 몰라요.
A: Why do you look so surprised?
B: I was reading a book, and the door suddenly opened and surprised me.
Yesterday I went to Jieun's housewarming party, and you have no idea how big her house is.
7. 우리 아이는 5살이 되고부터 얼마나 말을 안 듣는지 몰라요.
You have no idea how badly my boy doesn't listen to me since he turned five years old.
8. 책을 읽고 있는데 갑자기 문이 열려서 얼마나 놀랐다고요.
You have no idea how surprised I was when I was reading a book and the door opened all of a sudden.