有关客观真理
Do you know that the earth is a globe?
主句过去是,从句将来一个
Yesterday I forgot to tell him that the exam will start at 11am next Thursday.
主句过去,从句由现在完成改过去完成
By the time I went to the airport,I didn't realize that he had been waiting there for 3 hours.
主句一般现在,从句现在进行
Do you know that he is playing football downstairs?