Who would …? Who wouldn't …? ai mà, ai lại không

Some questions with ai (who) don't ask anything. Said with feeling, Ai mà biết! means "Who knows!", that is, nobody does. Mà after ai gives the question a "who on earth would …" sense, and the answer it expects is "nobody": Ai mà ngờ được! Who'd have thought it!

Add không, and the expected answer becomes "everybody": Ai lại không thương con mình? Who doesn't love their own child? Lại adds "of course", and mà works here too: ai mà không …. Often a topic comes first, closed by thì: Được nghỉ thì ai mà không thích? Who doesn't like a day off?

Nobody is meant to answer: agree, laugh or carry on talking. In Hà Nội you'll also hear ai chẳng or ai chả for "everybody": Ai chẳng biết! Everybody knows that!

Without mà or lại, a question with ai is usually a real one: Có ai biết đường không? Does anyone know the way? And ai cũng, which you know, says the same "everybody" calmly: Ai cũng thích được nghỉ. The question form says it with more feeling.