The book was written in 1946,the education system has witnes

D
since when = since1946,1946年是先行词,when是关系副词,
而since when用作副词短语,是从句中的时间状语.

这个句子可以这么拆分
the book was written in1946.
the education system has witnessed great charges since 1946这部分不缺成分,如果合并以后,就做1946的状语,因此用when,since 提前,1946 是先行词,合并,就得到你的原句


这么复杂的语法分析要是你看不懂的话,还有一个偏方：
后面是完成时的,你用since when
后面是过去式的,你用during when