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Last Christmas, the circus owner, Jimmy Gates, decided to take some presents to a children's hospital. Dressed up as Father Christmas and accompanied by a 'guard of honour' of six pretty girls, he set off down the main street of the city riding a baby elephant called Jumbo.
should have done：原本应该做而没做
needn't have done：过去不必做但是做了
should（应该）(表示建议、命令、要求)
ought to：应该（强调义务）
have to：不得不
所有的情态动词＋have done都跟过去有关
①跟虚拟语气有关的：would have done
②表示推测句型：may/must/can't have done
Though Jimmy agreed to go at once, Jumbo refused to move. Fifteen policemen had to push very hard to get him off the main street. The police had a difficult time, but they were most amused.
'Jumbo must weigh a few tons,' said a policeman afterwards, 'so it was fortunate that we didn't have to carry him. Of course, we should arrest him, but as he has a good record, we shall let him off this time.'
