时态一致表示句子的时态必须从头到尾都是一样的。即： 谓语动词的时态与时间状语的时态一致；从句的谓语动词时态与主句保持一致；直接引语的时态和间接引语的时态转换；
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.