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.

Structure

Time / Verb + 以前, ....

The time can be a specific time, or an action (technically, "when the action was done").

吃饭以前，你洗手了吗？Chīfàn yǐqián, nǐ xǐ shǒu le ma?Did you wash your hands before eating?
睡觉以前，不要吃东西。Shuìjiào yǐqián, bùyào chī dōngxi.Don't eat anything before you go to sleep.
两年 以前，你认识他吗？ Liǎng nián yǐqián, nǐ rènshi tā ma? Did you know him two years ago?
星期五 以前，你要做完这些工作。Xīngqīwǔ yǐqián, nǐ yào zuò wán zhèxiē gōngzuò.You need to finish this work before Friday.
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 ever 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 a house.
二十年 以前，这里是一个公园。Èrshí nián yǐqián, zhèlǐ shì yī gè gōngyuán.Twenty years ago, this was a park.
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 zhǎodào 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.Before he became the president, Obama didn't have white hair.

Note that this use of 以前 (yǐqián) can also be shortened to 前 (qián).