what's the difference between 羡慕 and 妒忌？
羡慕 is a good kind of envy. 

"You have a happy marriage. I'm really envy at you and I hope you can be happy forever" This is 羡慕.

妒忌 is a bad kind of jealousy. 

"You have a happy marriage. You don't deserve it! I should be happier than you!" This is 妒忌.