ㅗ it's like the "o" from Words or Hope ㅓ it 's like the "o" from Okay or Vowel ㅜ it's like the "ou" from You or the "oo" from Me too ㅡ i can't explain you the sound , because there is no such sound in english , but don't worry , this vowel is very difficult to pronounce. Put "E" on Google traduction (in french) and listen , this is sound but you need to tighten your teeth.

ㅗ - O in core ㅓ- British english : O in often American english: U in burn ㅜ - OO in moon ㅡ - like u in curt, pull (but lips not pushed out and not rounded)

I know what problems you have. That type of problems is found on many people depending on what the language is their mother tongue. In terms of vowels, you should know and practice exactly how to pronounce them. If you don't know what the difference of vowels between them, only listening will not help you realize what Korean people are saying. Speaking is a very good way to improve your listening. Small tip: you can make the sound of "누" when your upper tooth and lower tooth are in touch. But you can't make the sound of "너" at the time.

you have to hear the sound of these vowels . and listen to some examples. fighting 언니 ^^