I （like） these books very much.when were they（ bought）

题目

I （like） these books very much.when were they（ bought）
为什么可以这样填空?为什么是“when were they（ bought）”而不是“when did they bought?"

答案

这里是因为被动语态,
前面说我喜欢这些书,所以后面的they 指的就是书了,书只能被买,而且还是过去式,
所以是when were they bought 而不是when did they bought
被动语态就是be done