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。

例句：

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。

例句：

He thought that I need not tell you the truth.