by引导时间状语主句用什么时态
问题描述：
by引导时间状语主句用什么时态
答
一般用完成时.如果by 与过去时间连用,则主句用过去完成时.如：
We had finished learning five units by the end of last week.
如果by 与将来时间连用,则主句用将来完成时.如：
We will have completed this bridge by the end of tis year.
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