It turns out: hóa ra, thì ra

Hóa ra opens a sentence that tells you what turned out to be true, often something you didn't expect: Hóa ra anh Minh là anh họ của chị Lan. It turns out Minh is Lan's cousin. You can add là after it with no change in meaning: hóa ra là ….

It pairs naturally with tưởng, the "thought" that was wrong: Em tưởng quán đóng cửa luôn, hóa ra họ nghỉ trưa. I thought the place had closed down, but they were just on their lunch break. Tưởng ai, hóa ra là em! is a friendly greeting when someone turns up unexpectedly: "Oh, it's you!"

Thì ra means the same, with more of "so that's why": you've just understood something. On its own, Thì ra vậy! means "Oh, so that's it!" In Hà Nội it's Thì ra thế! Both words are as common in stories and the news as in conversation.

Hèn chi (no wonder) is the next step. Hóa ra gives the fact you've found out; hèn chi gives what it explains: Hóa ra hôm nay chủ nhật, hèn chi đường vắng. So it's Sunday. No wonder the roads are empty.