If (you) were to: person + mà + verb … thì

You know nếu … thì for "if". Speech has a shorter way: name the person or thing first, then mà, then the condition, then thì and the result. Em mà tới trễ thì chị giận đó. If you're late, I'll be cross (an older woman to a younger person).

It's more pointed than nếu, and most at home in warnings and in imagining: Em mà là anh thì em không mua. If I were you, I wouldn't buy it. Looking back, it carries regret: Em mà biết trước thì em không tới. If I'd known, I wouldn't have come. The verb doesn't change; the situation tells you whether it's real or imagined.

Mà goes after the person or thing, never first in the sentence the way nếu does: Trời mà mưa thì mình ở nhà. If it rains, we'll stay home. A sentence that starts with Mà means something else: "by the way" or "besides".