事故发生时我正在高速公路上开车。

The accident ___happened while I was driving on the highway.

我昨天碰巧在公园里遇见了我以前的老师。

I ___happened to run into my old teacher at the park yesterday.

昨晚那个派对上发生了什么有趣的事情吗？

Did anything interesting ___happen at the party last night?