睡觉前请把手机关机。

Shuìjiào qián qǐng bǎ shǒujī guānjī.

Please turn off your phone before sleeping.

老师让我们上课时关机。

Lǎoshī ràng wǒmen shàngkè shí guānjī.

The teacher asks us to turn off our phones in class.

如果你很忙，可以先关机。

Rúguǒ nǐ hěn máng, kěyǐ xiān guānjī.

If you are very busy, you can turn off your phone first.