--When __ you __ the bike.--Last month.

问题描述:

--When __ you __ the bike.--Last month.
填did;buy,为何have ;bought不行,句中有说到时态吗?

句子还原后面的回答是 I bought the bike last month ,有具体的过去时间状语,句子的时态就是过去时.
要是用have 的话就不是bought 而是 have had