must have done sth 一定做过某事 否定形式：can't have doneShe must have come here last night. 她一定是昨晚来的。She can't have gone there 她不可能到那儿去。
例:We do farm work as the old peasant teaches us.例:From what has been discussed above/taking into account all these factors， we may safely arrive at/draw/come to/reach the conclusion that the leisure life-style is undergoing a decline with the progress of modern society， it is not necessary a bad thing.
