They ____(learn)about eight hundred words by the end of last year.

问题描述:

They ____(learn)about eight hundred words by the end of last year.
have learned.
于是我希望您能自己回答...就这样
我是说为什么不是had learned

had learned by +过去时间 ,主句谓语动词用过去完成时态.谓语动词用 had +过去分词.