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■before 引導的時間狀語從句有時可用現(xiàn)在完成時和過去完成時來強調(diào)動作的完成。如：　　

Don’t get off before the train has stopped. 　　

He often gets off before the train has stopped. 　　

He often got off before the train had stopped.（可視為將the train has come變成過去，即成the train had come）

■若語義需要，before 的主句可以用現(xiàn)在完成時態(tài)。如：　　

The incense has never ceased to smoke before the tomb. 墳前煙火不斷。摘自《英語搭配詞典》　　

I have finished my dissertation before my supervisor went abroad. 在我的導師出國之前，我已完成了我的學位論文。摘自《薄冰實用英語語法詳解》　　

Before I realize it, I have eaten 10. 摘自《紐約時報》