But (unexpectedly), and yet: mà

Between two parts of a sentence, mà means "but" with a twist: the second part isn't what you'd expect after the first. Quán nhỏ mà đông khách quá. It's a tiny place, and yet it's packed. Vẫn (still) often follows it: Em học cả năm mà vẫn rớt. I studied all year and still failed (in Hà Nội, trượt).

At the start of a new sentence, vậy mà (in Hà Nội, thế mà) points back to what was just said: Ảnh hứa rồi. Vậy mà không tới. He promised. And yet he didn't come.

So mà carries a feeling: surprise, disappointment or admiration. Nhưng just sets two facts side by side. Phòng nhỏ nhưng sạch describes the room; Phòng nhỏ mà sạch quá sounds pleasantly surprised. In a neutral report or in writing, nhưng is the safer choice.

Mà has other jobs, and its place tells you which. At the end of a sentence it insists, like "I told you": Ngon mà! After a person, it can mean "if": Anh mà đi thì …