1) 如果從句所敘述的為真理或不變的事實，則永遠(yuǎn)用現(xiàn)在時。

　　At that time, people did not know that the earth moves.

　　He told me last week that he is eighteen.

　　2) 賓語從句中的助動詞ought, need, must, dare 時態(tài)是不變的。

　　He thought that I need not tell you the truth.