Unfortunately: tiếc là, rất tiếc

Tiếc is the feeling that something good didn't happen or can't: a pity, a shame. Tiếc là opens a sentence like "unfortunately": Tiếc là hôm đó em bận. Unfortunately I'm busy that day. Tiếc là trời mưa, mình không đi biển được. Shame it's raining, we can't go to the beach.

Rất tiếc is the polite version. It's what you'll hear from hotels, shops and offices when the answer is no: Rất tiếc, hôm nay hết phòng rồi ạ. I'm sorry, we're fully booked today. It says sorry without taking the blame; when something is your fault, apologise with xin lỗi. Em rất tiếc can also answer a friend's bad news, like "I'm so sorry".

On its own, Tiếc quá! means "What a shame!" or "Too bad!": Hết vé rồi hả? Tiếc quá! Sold out? What a shame! (in Hà Nội, à).

Like thật ra or nói chung, tiếc là tells the listener how to take the news before they hear it. Its opposite is may mà, "luckily".