时态一致

　　1) 如果从句所叙述的为真理或不变的事实，则永远用现在时。

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

　　He told me last week that he is eighteen.

　　2) 宾语从句中的助动词ought, need, must, dare 时态是不变的。

　　He thought that I need not tell you the truth.