Sometimes it shows an unexpected fact. Ex: 한국말 아주 잘 하는구만 (translation) You speak Korean very well. (Actually I doubted you can do that)

When you use 구만 it sounds like you're talking to yourself.

~구만 is a wrong form of ~구먼 ~구먼 has same meaning and usage with ~구나 Both forms are often said when one realizes a new fact or piece of information. The difference is in speech levels. ~구먼 is Hage-che(하게체) It is used only - by some older people when addressing younger people, friends, or in-laws in a friendly manner - between adult male friends, occasionally - in novels e.g. 자네는 돈이 별로 없구먼! ~구나 is Haera-che(해라체) It is used - to close friends or relatives of similar age, and by adults to children - in impersonal writing (books, newspapers, and magazines) and indirect quotations ("She said that...") - in grammar books, to give examples - in some exclamations e.g. 날이 밝았구나!

Sometimes it shows an unexpected fact. Ex: 한국말 아주 잘 하는구만 (translation) You speak Korean very well. (Actually I doubted you can do that)