定语从句中介词加关系副词的用法.比如from where

在北师大版高一新教材第3模块第7单元第1课课文中有这样的一句：He was blown off course and found himself in an unkown land,from where he eventually reached Greenland.
这里的where指的是in an land而不是land,因此只能用关系副词,而不用from which.再如：
He stood under the tree,from where he could see his son swimming.
You can go across the river,from where you can take a boat.

这个很简单，定语从句中，有介词的时候就用where 不用死扣语法

“介词+关系词”的用法
1、关系代词在从句中作介词的宾语时，介词常可以提前，但介词提前时，关系代词只能用which 或 whom。
如：The school (that/which) he once studied in is very famous.
→The school in which he once studied is very famous.
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