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

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

　　He told me last week that he is eighteen.

　　2) 賓語(yǔ)從句中的助動(dòng)詞ought, need, must, dare 時(shí)態(tài)是不變的。

　　He thought that I need not tell you the truth.