the book was written in 1946,{since when }the education syst

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the book was written in 1946,{since when }the education system has saw great changes.
逗号后面引导一个非限制性定语从句,以介词since开始,这不是介词+which的形式吗?为什么用when?选项since then为什么不对?

since then是副词短语,不能引导一个定语从句.
“The book was written in 1946”和“the education system has saw great changes”都是完整的句子,中间肯定是用连词连接的,副词没有这个功能.
定语从句有介词,一般我们都是用介词+which的,但是这个句子比较特殊,用一般的思路不好理解.我觉得可以这样想：
which在定语从句中是要充当成分的,像是主语或宾语.但是看后面的句子“the education system has saw great changes”,主谓宾是完整的.缺状语,那就用when,不用which了.

the book was written in 1946,{since when }the education syst 高三定语从句的题,1 The book was written in 1946,__the education syst 高手帮忙：That was written in 1946, since______the education syst The book was written in 1946,the education system has witnes The book was written in 1946,_______ the education system ha the book was written in 1946,-----the education system has s The book was written in 1946,( )the education system has wit T he book was writeen in 1978,(since when)the education stst The book was written in 1946,__the education system has witn The book was written in 1946,_____the education system has w ( )The book was written in1946,_____ the education system ha The book was written in 2001,___the education system has wit