Before a specific time with "yiqian"

As well as talking about the past in general, you can use 以前 (yǐqián) to talk about things that happened before a specific time.

吃饭以前，你洗手了吗？Chī fàn yǐqián, nǐ xǐ shǒu le ma? Before you eat, did you wash your hands?
睡觉以前，不要吃东西。 Shuìjiào yǐqián, bù yào chī dōngxi. Before you go to bed, don't eat food.
两年以前，你认识他吗？ Liǎngnián yǐqián, nǐ rènshi tā ma? Two years ago did you know him?
星期五 以前，你要做完这些工作。Xīngqī wǔ yǐqián, Nǐ yào zuò wán zhèxiē gōngzuò. Before Friday, you need to finish these work.
上大学 以前，你来过上海吗？Shàng dàxué yǐqián, nǐ lái guo shànghǎi ma? Before you went to college, did you come to Shanghai?
结婚以前，你应该先买房子。Jiéhūn yǐqián, nǐ yīnggāi xiān mǎi fángzi. Before getting married, you should first buy the house.
二十年 以前，这里是一个小村子。 Èrshí nián yǐqián, zhè lǐ shì yī gè xiǎo cūnzi. Twenty years ago, it was a small village here.
几个月 以前，他们分手了。Jǐ gè yuè yǐqián, tāmen fēnshǒu le. They broke up a few months ago.
毕业以前，我要找到工作。Bìyè yǐqián, wǒ yào zhǎo dào gōngzuò. I need to find a job before graduation.
当总统 以前，Obama 没有白头发。Dāng zǒngtǒng yǐqián, Obama méiyǒu bái tóufa. Obama didn't have white hair before he became the president.

Sources and further reading