적(的) is a suffix which attaches to a noun to create an adjective conveying a quality associated with the noun. It is one of the most common ways to create adjectives(the hanja-based ones), similar to "-ish", "-al", etc. in English. The -적 phrase can be used both in attributive ("효과적(인) 대책") and predicative role ("그 방책은 효과적이다"). Depending on the word's meaning, adding the prefix 비(非) (or 불(不) or 무(無) in some cases) gives you the antonym (like adding "un-" or "in-" in English), as in 비효과적이다 (is ineffective) = 효과적이지 않다 (is not effective). For example, - 기적(miracle) => 기적적(miraculous), 기적적(인) 생환(miraculuous survival/return), 기적적이다(is miraculous). - 현실(reality) => (비)현실적((un)realistic), (비)현실적(인) 대처((un)realistic response), (비)현실적이다. - 과학(science) => (비)과학적((un)scientific), (비)과학적(인) 접근((un)scientific approach), (비)과학적이다. - 비판(criticism) => (무)비판적((un)critical), (무)비판적(인) 논조((un)critical tone of discourse), (무)비판적이다. There is another 적 which is an unrelated native Korean word (의존명사, dependent noun) for "time" or "occasion", similar to 때 but with a narrower application. - 어릴 적에 살던 집 - the house I lived in during my childhood. - 공부할 적에는 집중해서 해야 한다 - You must concentrate when you study.

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