There are some frequently used Korean expressions that you will never learn from Korean books, Korean news or Korean newspapers. Some interjections, colloquial languages and newly-born expressions exemplify them. They are ambiguous in view of grammars; however, they have become so popular among people that they are very commonly used.
When Korean people need to think while speaking to someone, they say “음..(Um)”or “글쎄 (Geul ssae).” In English, these words are similar to “well…” The difference is that you often hear “well” from CNN while you hardly ever hear “Um” or “Geul ssae” from any of Korean news casts, because they are too colloquial. The phrase “I mean” can be translated as “그러니까 (Geu-Lu-Ni-Kka)” or “내 말은 (Ne-mal-Eun).” These phrases are also too colloquial to use in news or academic journals.
Compared to other languages, Korean has a lot of words and phrases to express one’s feelings. With regard to this fact, Korean has various interjections. An English word “very” can be translated as “진짜(Jin-Jja),” “정말(Jeong-Mal),” “제대로(Jae-Dae-Lo),””아주(A-Joo)”or ”참(Cham).” Thus, for example, you can say “Very good” in Korean in five different ways: 진짜 좋다!(Jin-Jja-Joh-Ta) 정말 좋다!(Jeong-Mal-Joh-ta) 제대로 좋은데! (Jae-Dae-Lo-Jo-Eun-De) 아주 좋아!(A-Joo-Joh-A) 참 좋아!(Cham-Jo-A)
Interjections used by teenagers are also various. “대박(Dae-Bak),” which used to indicate a big win from the lotto or a big success, now has general meaning of “very good.” For instance, as a reaction to beautiful scenes or to happy news, teenagers will say “와, 대박이다! (Wow! Dae-Bak-Yi-Da!)” A word “짱(zzang)” has two very different meanings: one is an abbreviation form of “짜증(zza-jeung)”which means annoyance, and the other is “best.” So, “정말 짱난다(Jeong-Mal-Zzang-Nan-Da)” will be translated as “It’s very annoying!” while “넌 짱이야 (Nun-Zzang-Yi-Ya)” will be “You are the best!”
Many colloquial expressions including those mentioned above are hard to learn from books, but they are very commonly used in Korea. Thus, I would suggest you study them from Korean movies or dramas. Korean movies and dramas will be both fun and helpful.