1 sentence

1. 听到老师走进来，他立刻挺直了腰板。

   tīng​dào lǎo​shī zǒu​jìn lái, tā lì​kè tǐng​zhí le yāo​bǎn.

   Hearing the teacher come in, he immediately straightened his back.

   Source: Dong Chinese Dictionary