어느새 => 어느(which, what) 사이(span of time)에 => before I knew it. * 새 = 사이: a distance or span of time between two points in space or time. * 어느새 is a contraction of 어느 사이에, literally "in some (unknown) time period (which has already passed)". For "suddenly", we have a different group of words like 갑자기, 갑작스레, 급히, etc.

어느새 means rather "before someone realized it" than "suddenly" 비가 어느새 그쳤다 The rain stopped before we knew it. 그 코흘리개가 어느새 자라서 어른이 되었다 The little kid has already grown up to be an adult. 어느새 여름방학이 다 지나갔다 The summer vacation has passed all too soon[has simply flown by]. 입학한 것이 엊그제 같은데 어느새 졸업을 하게 되었다 It seems like yesterday that we started the school, but we are graduating already. 어느새 비가 내리고 있었다 It had started raining before I was aware of it[I realized it]. 그는 어느새 없어졌다 He had slipped away.