太阳落山以后，天气一下子变凉了。

Tàiyáng luòshān yǐhòu, tiānqì yíxiàzi biàn liáng le.

After the sun set, the weather suddenly became cool.

别把书落在教室里了。

Bié bǎ shū là zài jiàoshì lǐ le.

Don't leave your book behind in the classroom.

比赛中他一不小心就落在别人后面了。

Bǐsài zhōng tā yì bù xiǎoxīn jiù luò zài biérén hòumiàn le.

During the race, he carelessly fell behind others.