恰巧 (qià qiǎo): โดยบังเอิญ

Yesterday I was looking for a hard-to-find book in a bookstore and happened to run into an old classmate I hadn't seen in years; we were both pleasantly surprised.

He was originally going to Shanghai on a business trip, but it just so happened that the airline canceled that flight, so he took the train instead and actually arrived early.

We were just discussing where to go for the weekend, and it happened that a TV program about West Lake in Hangzhou was on, so everyone immediately decided to go to Hangzhou.

She happened to call just when I needed help the most, asking how I had been lately, which made me feel especially warm.

When the police retrieved the surveillance footage, they happened to discover the suspect leaving through the back door of the convenience store, and thus quickly pinpointed his whereabouts.