- 놀이터 is for children, while 운동장 is..... well 運動場 - 自動車 vs 自家用. I hope you understand better now as a Japanese. - 성인(成人) is more of a legal term while 어른 means adult in general.

- 놀이터: 놀이(play, game) + 터 (ground, space). Playground (for children). - 운동장: 운동(exercise, sport) + 장(ground, lot). Exercise and sports ground (in schools and public parks, etc) - 자동차: 자동(auto) + 차(vehicle). Automobile in general. - 자가용: 자가(self) + 용(usage). "Private usage" (automobile, boat, airplane, etc). Ex: 자가용 차: private car, 자가용 비행기: private airplane. - 성인: adult. Technical/legal term used in statistics, law, etc. - 어른: adult. General term. Also has another meaning: a respected elder.