There are two main categories of words in Korean. One is pure Korean word(고유어). The other is Sino-Korean word(한자어:loan word from ancient Chinese) The pure Korean word can be used alone. Sino-Korean word is not used alone but used by compound word with another Sino-Korean word. 물 vs 수 물 and 수 both mean "water" 물 is pure Korean word 수 is Sino-Chinese word(loan word from ancient Chinese : 水) 냉수(冷水) : cold water 온수(溫水) : warm water 탄산수(炭酸水) : soda water 물 can be used alone I drank water 물을 마셨어요.(O) 수를 마셨어요.(X) I drank cold water) 찬 물을 마셨어요. (O) 냉수를 마셨어요. (O) 목 vs 나무 목 and 나무 both mean "tree" 목 is Sino-Chinese word(木) 나무 is pure Korean word 초목(草木) : grass and tree 수목(樹木) : tree There were densely packed trees in the park 공원에는 나무가 울창했다(O) 공원에는 목이 울창했다(X) 공원에는 수목이 울창했다(O)