while he was reading the letter,he heard a knock at the door

这句话应该把while改成when.while应该接的是两个可延续的动词,例如,While she was reading,he was playing computer games.而你的句子中hear这个词听到敲门声是断断续续,且听到每一声都是瞬间完成的,不是延续的.when可以接两个不可延续的动词,也可以接一个可延续,一个不可延续.