No wonder: hèn chi / thảo nào

When something suddenly makes sense, start with hèn chi: "no wonder", "that explains it". What follows is the thing you now understand, and the reason has usually just been said: Hôm nay chủ nhật hả? Hèn chi đường vắng. It's Sunday? No wonder the roads are empty.

Hèn gì means the same and is just as common in Saigon: Chị ấy ở Mỹ lâu rồi, hèn gì giỏi tiếng Anh vậy. She lived in America for years, no wonder her English is so good. In Hà Nội people say thảo nào: Thảo nào đường vắng thế. You'll be understood with any of them.

On its own, Hèn chi! is a whole reply: "That explains it!" or "Ah, so that's why!" It's a friendly way to show you've followed what someone told you.

Vậy at the end (in Hà Nội, thế) often adds surprise: hèn chi đông vậy, no wonder it's so crowded. For a reason you're giving yourself, rather than one you've just discovered, use vì or tại vì.