3、不必”,mustn't表示“禁止,不允许”e.g.—MustI finish all assignments at a time?—Yes,youmust.No, youneedn't.Youmustn'tget off while the car is still moving.must+havedone表示对过去已经发生的行为进行推测,意为“想必/准是/一定作了某事”。1.Itmusthaverainedlastnight,fortheroadwasverymuddy.昨晚一定是下雨了,因为地上很泥泞。2.Wemusthavelearned2,000wordsbytheendoflastt