You meant "못" in ~하다 , ~ 먹는다, etc.? Cause just word,못 stands for a nail :P So I think you want to get the difference between 못 ~ and ~ 수 없다, don't you? Like 못 먹는다. and 먹을 수 없다, 못한다. and 할 수 없다. Yeah 못 is used an adverb with a verb meaning negative. But it's different to 안 meaning also negative in the point that 못 verb is for not being able to do sth, however 안 verb is for not doing sth 난 사과 안 먹어 I don't eat apples 난 사과 못 먹어 I can't eat apples 못~ and ~ 수 없다 are used the same way to show whether sb/sth is able to do sth. 난 거기에 못 간다. 난 거기에 갈 수 없다. Both say I can't go there, but 수 없다, short form of 수가 없다 has stronger meaning, because 수 is a word meaning "a way", so technicaly the second sentence says I don't have any way to go there. 그 사과는 못 먹는다. 그 사과는 먹을 수(가) 없다.