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1．表示按计划发生的动作时，两者可互换。
We are moving to a new flat tomorrow.
我们明天就要搬到新房子里去了。
We are going to move to a new flat
tomorrow.
我们明天就要搬到新房子里去了。
2．表示由于客观因素而产生的将来动作或状态时，用be going to结构，不用be + 动词的-ing形式。
You are going to fall if you climb that
tree.
如果你爬树的话，你会摔下来的。（不可说 You are falling if ...）
Be careful. You are going to break that
chair.
当心！你会把那张椅子弄坏的。（不可说 You are breaking that chair）
③ be + 动词不定式
这一结构中的be，只有现在式 (am, is, are) 和过去式 (was, were) 两种形式。
a. 表示按计划或安排将要发生的动作
The highway is to be open to traffic in
May.
这条公路将在五月份通车。
Am I to take over his work?
我是不是要接管他的工作？
b. 用于条件句中强调按计划或安排将要发生的动作
If we are to take the 5:00 train, we
must leave now.
如果我们乘坐5点的火车的话，那我们现在就得出发。
c. 表示命令或吩咐等。否定形式常表示“禁止”或“不许”。
You are to be back before 10 p.m..
你必须在10点前回来。
You are not to go out alone at night.
晚上你不能单独出去。
比较：
“be + 动词不定式”与“be going to+ 动词原形”的区别
1．“be going to+ 动词原形”侧重说话人个人的意图和打算，“be + 动词不定式”侧重受别人的指示或安排要做的事。
I'm going to try my best to write this
article well.
我将尽力把这篇文章写好。
Am I to wait here till their arrival?
我要在这儿一直等到他们抵达吗？
2．表示由于客观因素或不受人控制的将要发生的动作时，只用“be going to+ 动词原形”，不用“be + 动词不定式”。
It's going to rain.
天要下雨了。（不说It's to rain.）
Rachel is going to faint.
雷切尔要晕倒了。
④ 一般现在时
一般现在时可以用来表示将来时间，主要用法有：
a. 表示由于日历或时刻表的规定而固定不变的或比较不易变更的将来时间发生的动作。
Tomorrow is Friday.
明天是星期五。
What time does the next train leave
for Paris?
下一班开往巴黎的火车几点出发？
b. 在时间状语从句或条件状语从句中，用一般现在时表示将来时间的动作或状态。
I'll give the book to you after I finish it.
我看完这本书就给你。
If he arrives, we must go and meet
him at the railway station.
如果他到了，我们就得到火车站去接他。
c. 在hope, suppose等后面的宾语从句中常用一般现在时表示将来动作或状态。
I hope all is well with him.
我希望他一切都好。
Suppse we go hiking tomorrow.
我们明天还是去远足吧。
D．一般过去将来时
1．一般过去将来时的构成
一般过去将来时是由“should/would + 动词原形”构成的。
He asked me yesterday when I should
leave for Paris.
昨天他问我什么时候动身去巴黎。
They wanted to know how they would
finish the homework earlier.
他们想知道怎样才能早一点儿完成家庭作业。
2．一般过去将来时的用法
一般过去将来时间的出发点是过去，即从过去某一时刻看以后要发生的动作和状态。
a. 一般过去将来时常用于间接引语中
He said they would arrange a party.
他说他们将安排一个晚会。
I asked if he would come and fix my TV
set.
我问他是否来帮我修电视机。
b. 一般过去将来时可用来表示非真实的动作或状态
If I had a chance to study abroad, I
would study at Cambridge University.
如果我有机会出国学习的话，我就会去剑桥大学。
I wish he would go with me to the
cinema tonight.
今晚他能和我一起去看电影就好了。
3．过去将来时间其他表达法
a. was/were going to +动词原形
He said that he was going to live in
the country when he retired.
他说他退休后要住在农村。
They thought it was going to rain.
他们认为天要下雨了。
b. was/were +动词的-ing形式
Nobody knew whether the guests were
coming.
没人知道客人们是否要来。
I was told that the train was leaving
in a few minutes.
我被告知火车几分钟后就要开了。
c. was/were +动词不定式
She said she was to clean the
classroom after school.
她说她放学后要打扫教室。
It was reported that another bridge
was to be built across the Yangtse River.
据报道长江上将要再建一座大桥。
提示：
“was/were going to +动词原形”或“was/were + 动词不定式完成式”可表示未能实现的过去将来时间的动作。
Last Sunday we were going to visit the
Great Wall, but it rained.
上星期天我们本想去游览长城的，但却下雨了。（没有去成）
I was to have helped with the
performance, but I got flu the day before.
我是打算帮忙演出的，但前天我感冒了。（没有帮上忙）
d. was/were about to do
“was/were about to do”表示说话的瞬间就会发生的动作。
I felt something terrible was about to
happen.
我觉得有可怕的事要发生了。
e. was/were on the point of doing
I'm glad you have come. I was on the
point of calling you, but you've saved me the trouble now. 很高兴你来了。我正准备给你打电话，现在你省去我这个麻烦了。
提示：
“be about to do”和“be on the point of doing”结构一般不与表示将来的时间状语连用，但后面可以接when引导的分句。
I was about to start when it suddenly
began to rain.
我正要动身天突然下雨了。
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