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.