问题描述：
since 与 before 的区别?
Mr.Green has lived in China since I came here Mr.Green has lived in China before I came here 第一个正确吗?before是不是也可以用于完成时态?
Mr.Green has lived in China since I came here Mr.Green has lived in China before I came here 第一个正确吗?before是不是也可以用于完成时态?
问题解答：
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