51. He must have had an accident, or he _A_ then.

　　A would have been here B had to be here C should be here D would be here

　　must have + 過去分詞 表示對過去行為的肯定推測;

　　與過去事態(tài)相反用：情態(tài)動詞 + have + 過去分詞 來體現(xiàn)虛擬語氣;

　　must 一定，必須; needn't, don't have to 不必;

　　53. You _A_ all those calculations! We have a computer to do that sort of thing.

　　A needn’t have done B must not have done

　　C shouldn’t have done D can not have done

　　should have + 過去分詞 本應該，本應當; shouldn't have done本不應該，本不應當;

　　44. It was very kind of you to do the washing-up, but you _D_ it.

　　A mustn’t have done B wouldn’t have done

　　C mightn’t have done D didn’t have to do

　　52. It was essential that the applications forms _C_ back before the deadline.

　　A must be sent B would be sent C be sent D were sent

　　It be essential that 后面一定用虛擬語氣，且句子謂語動詞固定為(should) + 動詞原型;

　　It was essential that the applications forms send back before the deadline.

　　53. We _D_ our breakfast when an old man came to the door.

　　A just have had B have just had C just had D had just had

　　when 還可以表示①剛..就… (有動作先后關(guān)系)、②恰在此時;

　　44. I had just started back for the house to change my clothes _D_ I heard voices.

　　A as B while C after D when

　　45. I felt somewhat disappointed and was about to leave, _C_ something occurred which attracted my attention.

　　A unless B until C when D while

　　64. It was essential that these application forms _D_ back as early as possible.

　　A must be sent B will be sent C are sent D be sent