before引导时间状语从句。
before：
prep.之前;在…以前;在…面前(或前面);(次序或排列)在前面。
conj.在…以前;到…为止;到…之前;以免;不然。
adv.以前;过去;已经。
I'd like a second opinion before I make a decision.
我在作决定之前，想听听别人的'意见。
I didn't have time to change clothes before the party.
我没时间在聚会前更换衣服。
It would be a good idea to call before we leave.
我们出发之前打个电话是个好主意。
Profits were at the same level as the year before.
利润和前一年的持平。
Have you ever used this software before?
你以前用过这种软件吗？
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