1.表示将来的时间.
主句用将来完成时,而从句用一般现在时来表示将来的动作.
如:I will have finishied my homework by the time my father comes back.
2.表示过去的时间.
主句用过去完成时,而从句用一般过去时.
如：He had left by the time we reached home.