The difference between them is the tone, man. Different Vietnamese accents have different tones. It's quite hard to explain by just writing, you'd better learn it through listening (it's even better if you have someone who is Vietnamese practice with you). Anyway, you have no choice but try to distinguish them (by watching the video mentioned above). I know it takes time, but just be patient. Sooner or later you will make it! Feel free to ask if you have any further question or even want me to help you with that problem. Good luck!

Hi Josh, in the North, people clearly distinguish those 2 tones but in the South (like your mother's accent) we do not, we speak the same and we still understand each other. When we have chance to have voice chat on skype, I'll show you :)

add friend and I can help you

As Tung said, the difference between them is the tone. You can learn it easier if you practice them with the northern people (Ha Noi for example). And a hint for you, in writing they are difference although some people pronounce them as the same. ;) Goodluck.

không có quy luật nhé bạn. Practise makes perfect !