믈 vs. 스
I understand that they are both water.

믈 = water to drink?
스 = water in a lake or to swim in?

Also: 목 vs. 나무
Both tree? Is there a difference?

감사합니다

Question

믈 vs. 스
I understand that they are both water.

믈 = water to drink?
스 = water in a lake or to swim in?

Also:  목 vs. 나무
Both tree?  Is there a difference?

감사합니다

Answer

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)