"She is sad but he is not serious."
Translation:Cô ấy đang buồn vậy mà anh ấy không nghiêm túc.
I would really like an explanation on when/where to use nhưng, mà, and vậy mà. It seems in this course this is the only module that allows you to use mà or vậy mà for "but", if you try doing it in any other module, no matter how similar the construction seems to be, you get zapped.
So either there is some unexplained difference in usage, or mà/vậy mà is being unnecessarily penalized as wrong in these other instances.