要素3：时态 ①主句是现在的时态，从句的时态可根据实际情况而定。 主现从任 I hear that she has written three letters to the headmaster. Mr Green says that the snow was very heavy last month. I don't know who knocked at the door so loudly.要素3：时态 ①主句是现在的时态，从句的时态可根据实际情况而定。 • I hear that she has written three letters to the headmaster. • Mr Green says that the snow was very heavy last month. • I don’ t know who knocked at the door so loudly. 主现从任