11.27 时态一致
        
        　 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.
        
        11.28 时态与时间状语
        
        　　　　　　　　　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