简单的说就是站在过去的角度说将来会怎么样.一般以I tought,He said, They hoped..之类的句型开始,也就是说一般主句的谓语要是过去时.举例：I thoguht I could have finished it in two days. (我原以为我两天可以完成的.)He promised he would have his room cleaned before his mother came back. (他说他将会在妈妈回来之前把房间打扫干净.)
也可以是虚拟语气,就是对过去的事情的假设.
举例：I should have done it earlier, it's too late now. （我本应该早点做的,现在太晚了.）