语法：时态一致、现在进行时替代将来时

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.

现在进行时替代将来时

1) 表示即将发生的或预定中计划好的活动。
Are you staying with us this weekend? 这周和我们一起度周末吗?
We are leaving soon.　我们马上就走。
2) 渐变动词，如：get, run, grow, become, begin及die。
He is dying.