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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.
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