过去完成时。

表示"过去的过去"，例如:

They had lived in China for three years by the end of last month.

到上个月为止他们已经居住在中国三年了

还可以用在宾语从句中，例如:

He told me ( that) he had lived in Beijing since he left from his hometown.

他告诉我说他自从离开家乡就住在北京