~구만 = Incorrect but widely used form of ~구먼

~구먼 has same meaning and usage as ~구나 but

~구먼 is 하게체 (familiar speech level), used by older people, but not to children, in novels, and between adult male friends. 

기가 막히는구만 = I can’t believe this.

자네는 돈이 별로 없구먼! = You (younger man) have barely any money! 

유능한 젊은이 처럼 보이는 구만 = Seems like a capable young man 

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~구나, 군, 군요 = You have just found out something you didn’t realise or know before 
~았/었구나, ~ㄹ/을 것이구나, ~ㄹ/을 거구나 
~

는구나 after present tense action verbs

어제는 저의 생일이었어요 – Yesterday was my birthday
어제 생일이었구나 – Oh! I didn’t know that yesterday was your birthday과학선생님이군요! = Oh! I didn’t know that you are a science teacher.
벌써 먹었구나 = Oh, I didn’t know that you already ate
네가 매우 똑똑하구나 = I didn’t realise that you are so smart
아~ 학생은 책을 안 가져왔구나 = Oh, you didn’t bring your book (I just realised that you didn’t bring your book)

슬기는 어디에 있어? = Where is Seulgi?
슬기는 이미 집에 갔어 = Seulgi already went home
그렇구나 = Oh.. I see, I didn’t realise that before

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~(ㄴ/는)다/더라(고요) = somebody said, apparently, they say that (information you read/heard somewhere else)
(contracted from ~다/라고 하더라)

야, 미국은 좀 그렇고 캐나다가 좋다더라 – Say, I heard that the US was not that great but that Canada was nice.
철수가 요리를 못한다더라 – (I heard that) cheolsu is not good at cooking.
말레이시아는 한국에서 멀다더라 – Malaysia is far from Korea.
달에는 생명체가 없다더라 – There is no living creature on the moon.
그녀는 말레이시아인이라고 하더라 – 그녀는 말레이시아인이라더라 – (someone said that) She is Malaysian.

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~더라 (younger and close relationship), ~던데(요) (polite) = something to realised after experiencing/seeing/hearing about it 

(similar to ~구나 but you realised it in the past)

그 영화 어제 봤는데, 재밌더라! = I saw that movie yesterday, and it was fun!
어제 경화 씨를 만났는데, 머리를 염색했더라 = I met Kyung-hwa yesterday and she had dyed her hair
싱가폴에 처음 가 봤는데, 정말 덥던데요 = I went to Singapore for the first time, and it’s really hot there.
윤아 씨한테 물어봤는데, 모르던데요 = I asked Yoona, and she doesn’t know.

~더라 is a little excited/exclamatory, ~더라고(요) is more calm, neutral

아까 효진 씨를 만났는데, 남자친구랑 있더라고요 = I met Hyojin earlier, and she was with her boyfriend

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~더라는데 – it has been observed that, as we know, I hear (have been told) that, I noticed that
(Contracted from -더라[고 하]는데)

웃더라는데 (학마을 ) – they said that [she] was smiling