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

时态与相对应的时间状语

一般现在时　　every …, sometimes,　at …, on Sunday,

一般过去时　　yesterday, last week, an hour ago, the other day, in 1982, just now

一般将来时　　next…, tomorrow, in+时间,

现在完成时　　for, since, so far, ever, never, just, yet, till/until, up to now, in past years, always, recently

过去完成时　　before, by, until, when, after, once, as soon as

过去进行时　　this morning, the whole morning, all day, yesterday, from nine to ten last evening… when, while

将来进行时　　soon, tomorrow, this evening, on Sunday, by this time, tomorrow, in two days, tomorrow evening