几个英语的虚拟语气的问题不懂,请高手指教.WhenhetookthepenbacktoMary,shesaidcoldly,"You_itsooner,forI_it."正解：shouldhavereturned;mighthaveneeded为什么前面用shouldhavereturned?既然前面是这个时态,

　　几个英语的虚拟语气的问题不懂,请高手指教.

　　WhenhetookthepenbacktoMary,shesaidcoldly,"You_itsooner,forI_it."

　　正解：shouldhavereturned;mighthaveneeded

　　为什么前面用shouldhavereturned?既然前面是这个时态,为什么后面还是现完时?

　　Iwouldhavewrittenbefore,butI_illthen.

　　正解：was

　　这句话怎么理解?用hadbeen怎么错了?咋用was呢?

　　Supposeyou_himnow,whatwouldyousaytohim?

　　正解:met

　　还是时态问题搞不清,为什么用met啊?

　　It_amistakenottohelphim.

　　正解：wouldbe

　　这句话怎么判断是对过去虚拟还是对现在虚拟啊?