他恰巧坐在我旁边，我们就聊了起来。

Tā qiàqiǎo zuò zài wǒ pángbiān, wǒmen jiù liáo le qǐlái.

He happened to sit next to me, so we started chatting.

我们在国外旅游时，恰巧遇到了大学同学。

Wǒmen zài guówài lǚyóu shí, qiàqiǎo yùdào le dàxué tóngxué.

While we were traveling abroad, we happened to run into a college classmate.

今天恰巧是他的生日，我们一起给他庆祝吧。

Jīntiān qiàqiǎo shì tā de shēngrì, wǒmen yìqǐ gěi tā qìngzhù ba.

Today happens to be his birthday, so let’s celebrate it together.