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be to do共有八种用法：表示坚决的命令、表示“计划”或“安排的事项”、表示“可能性”、表示“意图”、表示“应该”、表示“注定”“不可避免”“必然发生”、用于虚拟语气，表示一种假设、有“期待将来”的含义。
be to do 的用法详解
1.表示坚决的命令，可译为“必须”。
If you are to be there on time, you had better hurry.要想准时到达那儿，你最好快点。
(2)If all of us are to remain friendly, we must be sincere with one another.要想保持友谊，我们都必须真诚相待。
5.表示“应该”。
(1)You have done so well. You are to be rewarded.你做的很好，你应该受到奖赏。
(2)Such nasty questions are to be avoided.你应该避免这种下流的问题。
(3)You are to call the police.你应该报警。
6.表示“注定”“不可避免”“必然会发生”。
(1)Anne was wondering what was to become of her boy friend, Tommy.安妮不知道她的男朋友托米会出什么事。
(2)The murderer was sentenced to life imprisonment and was never to see his family again.杀人犯被判终身监禁并且永远不能和家人见面。
(3)She did not think too much of her unkind acts.However，they were to be harmful to herself，too.她对她不友好的行为并没有想太多，然而，这些行为必将给她带来不利。
(4)Tom wasn’t worried about his head injury,but it was to be troublesome for him later.汤姆不担心他头部的伤，但是以后这损伤必将给他带来麻烦。
(5)You are to answer for what you have done.你必将为你所做的事承担后果。
7.用于虚拟语气，表示一种假设。
(1)If I were to do it, I would do it well.这件事如果我来做，我会做好它。
(2)Even if you were to take a taxi now, you would not be able to arrive at your destinaton on time.现在即使坐出租车，你也不会准时到达目的地的。
8.有“期待将来”的意思。
(1)The actual usefulness of the newly invented equipment is still to be determined.这种新发明出来的设备的实用性如何还有待观察。
(2)Henry's appointment with the accountancy firm is yet to be confirmed.亨利与这家会计事务所的约定还有待确认。
be to do和be going to的区别
1、am/is/are going to 加动词原形，表示主语的意图或打算将来做的事。如：
What are you going to do tomorrow? 你打算明天做什么呢？
The play is going to be produced next month. 这出戏下月开播。
另外：已有迹象表明将要发生的事，只能用 am/is/are going to+动词原形。如：
Look at the dark clouds; it’s going to rain. 看那乌云，快要下雨了。
2、be to加动词原形，仅用于表示正式的公务安排、公告指示、义务、禁止等。如：
You’re to deliver these flowers before 10. 你在10点钟之前把这些花送去。
You are to (=should) report to the police. 你应该报警的。
You’re not to (=mustn’t) tell him anything about our plans. 你不要把我们的计划透露给他。
注：这与不定式作表语不同：My idea is to go there today. 我的意思是今天就去那里。
3、be to强调客观安排或受人指示而做某事，be going to则强调主观的打算或计划。
4、be about to加动词原形，表示即将或马上要做某事。如：
He is about to leave for Beijing. 他马上要去北京。