overheard one's talking 偶然听说 Overhear v. 无意中听到；偷听。
常见句型有①S+～+ n./pron.②S+～+ n./pron. +to/ -v.③S+～+ n./pron. + v -ing，如I overheard them talking.

【例句】
I was too far away to overhear their conversation.
我离得太远了，听不见他们说什么。

I overheard him leave for Beijing.
我偶然听说他要到北京去。