Verb/Event + 时
时 (shí) means "time" or "at the time of" and functions as a time-reference word in Chinese. When placed after a phrase or clause, it indicates "when" or "at the moment that." For example, 吃饭时 means "at the time of eating" or "while eating."
吃饭时不要看手机,对眼睛不好。
Chī fàn shí bú yào kàn shǒujī, duì yǎnjīng bù hǎo.
Don't look at your phone while eating; it's bad for your eyes.
上课时请把手机调成静音。
Shàng kè shí qǐng bǎ shǒujī tiáo chéng jìngyīn.
Please set your phone to silent mode during class.
下雨时,街上的人都在跑。
Xià yǔ shí, jiē shàng de rén dōu zài pǎo.
When it rained, everyone on the street was running.