벌써, 이미, 어느새 (and 어느덧)
They are related and the difference is sometimes small, but they do have different usages too.
- 벌써: earlier than the time expected. (has an element of surprise)
- 이미: before the present time. (emphasizes the unchangeable nature, with no sense of surprise)
- 어느새: happening before one notices it. Before the person knows/knew or realizes/realized.
- 어느덧: as time went by. Similar to 어느새, but more relaxed (less of a surprise) and literary.
새 (contraction of 사이) means a duration between two points in time.
어느새 is short for 어느 사이에, which is literally "during an unnoticed passage of time".
덧 means time as a fast moving, elusive entity, so 어느덧 signifies "(already) at a new unfamiliar time").
Examples will make it clearer (alternative choices are given in parentheses):
- 이런, 벌써 여덟 시다. Gee, it's already eight o'clock. (어느새)
- 그가 죽은 지 벌써 오년이나 됐다: It's already 5 years since he died (어느새, 이미).
- 이미 엎질러진 물이다: It's spilled milk already. (벌써)
- 이미 지난 일이니 너무 상심하지 마: Don't bug yourself with what's already past. (벌써)
- 우리들도 어느새 성장해서 어른이 되었다: Before we realized we have all grown to be adults too. (어느덧, 벌써)
- 하늘엔 어느새 달이 떠 있었다: Before I knew the moon was already up in the sky. (어느덧, 벌써))
- 세월은 흘러 그들도 어느덧 노인이 되었다: Time passed and they now found themselves in old age. (어느새, 벌써)