唉 (ài): alas, oh

Ugh, how did I lock my keys inside the apartment again!

Sigh, I waited in line for an hour, and in the end the doctor said they couldn't take any more appointments today.

Sigh, Dad let out a sigh and said business hasn't been good this year, so he probably won't be able to take us to Hainan for the New Year.

Oh no, I got even such an easy question wrong; how am I going to explain this to my mom when I get home?

Come on, don't be sad; you didn't do well this time, but there's still a chance next time.