大家都很着急，这时老师走了进来。

Dàjiā dōu hěn zhāojí, zhè shí lǎoshī zǒu le jìnlái.

Everyone was very worried; at this moment, the teacher walked in.

我正要出门，这时突然下雨了。

Wǒ zhèng yào chūmén, zhè shí tūrán xiàyǔ le.

I was just about to go out when it suddenly started to rain.

他们在讨论作业，这时老师走进来了。

Tāmen zài tǎolùn zuòyè, zhè shí lǎoshī zǒu jìnlái le.

They were discussing homework when the teacher walked in.