为您解答可以填,只不过和when略微不同。在非限定定语从句里,介词+when=介词+then,when做状语,关系副词介词+which=介词+that,which是关系代词这里你说before then I was reading....和before that I was reading...都是可以的。
为什么不能填呢?个人认为就应该填before which(介词+which引导非限制性定语从句:which代表ten o'clock)(尽管用得不太多,但还是可接受的。)(不过,这个句子确实有点怪:改为这样更好:I went to bed at ten after I (had) read for an hour.)用过去进行时真的不好。又如:He has been a sales assistant here for nine years, before which he was a makeup artist. 作为销售助理他已经在这里工作了9年。 在这之前他是个化妆师。(例句来自有道词典)