힘듬 is hard or tough 어려운 is difficult 곤란한 is also difficult but usage is a little different. when you have some trouble caused by some situation, you use "어려운 and also 곤란한" for exapme... A: 돈좀 빌려줘 B: 지금은 어려운데, 지금은 곤란한데 are both correct because there might be some situation that you can't lend your money but you don't use '곤란한" for things.. for example "this math question is difficult" "이 수학 문제는 어렵다" is right "이 수학 문제는 곤란한다" is strange sentence... there is no reason or situation... Math is just difficult.. =,=;;

I've found that a good way to know which one to use is to follow these guidelines. 힘든 = physically difficult, hard work = 힘든 일 어려운 = mentally or emotionally difficult, 어려운 숙제 = hard homework. 어려운 사정 = hard circumstances 곤란한 can be used just like the second example for 어려운, but I rarely hear it used.