Even more important than hanja or pure Korean origin is the fact 신 and 구 are mostly used as prefixes (접두사). The pure Korean adjectives for 신, like 새로운 and 새 are full, general words. In contrast, 신 is not used as a full adjective but attaches to other hanja-based words to create many new words conveniently. Examples will illustrate this. * 새, 새로운, vs 신 - 새 차를 샀다. "Bought a new car"; "신차를 샀다." can also work. - 새로운/새 날이 밝았다. "A new day broke."; You never say "신 날이 밝았다". - 새 세대. "new generation"; 신세대 (one word) means the same, probably used more often. - 신세계. "new world" - used much more widely than "새 세계". - 차를 신형으로 샀다. 신형 (new model) is shorter and more common than the comparable "새 모델". * 지난/지나간 (past), 오래 된 /오랜/옛(old) vs 구 - 지나간 날들 (bygone days); you don't say "구 날들". - 옛이야기: special case where "옛" is a prefix; you never say "구 이야기". - 차가 지나간 모델이다 = 차가 구형이다. (구형 = old model/type) - 구체제 = 옛 체제 = old system / institution - 구세대 = 지난 세대 = old generation You can see that 신/구 works as a prefix for hanja-based words like 차, 세계, 세대, 형, 체제, etc. to create new words that are just one syllable longer. The pure Korean words are more general but in most cases adds a full word and not as convenient for hanja-based words. It is a case of hanja-originated words working better with other hanja-based words. But the above general rule has exceptions to make things more difficult. For example while 신차 is commonly used, no one uses the opposite 구차. 구닦다리 is a common word for "old fashioned", "outdated" which combine 구 with a pure Korean word.

신(新) is a Hanja (the Korean name for Chinese characters) written phonetically with Hangul (Korean alphabet). 새롭다 is a pure Korean word. The Korean words based in Hanja is mainly used in formal and academic writings.