例如：
He is going to be a doctor.(么得把握滴，表一种愿望)
He is to be a doctor。（或者现在从医学院毕业了，马上就去做医生了）
参考
一般说来，“be to do”这个结构有两种语法意义，其一是连系动词be+动词不定式做表语，其二是be to是一个独立词汇单位,具有情态含义,可以把它叫做情态习语(modal idiom)。
一、be +动词不定式,不定式做表语,表示主语和表语在概念上是等同的。如：
The problem is to find a solution.
His plan is to clean the room.
My wish is to be a doctor.
二、be to +动词不定式中的be to用作情态习语,这时的be to do表示: “计划”、“安排”、“义务”、“应该”、“可能”、“命运”等。(have to, ought to)。如:
He is to have a holiday. (表示将来)
The committee is to meet today. (表示计划、安排)
You are to go to the hotel where rooms have already been booked for you.
1.表示“将”、“计划”、“安排”。(意思接近于be going to)如：
Their daughter is to get married soon.
Who is to question him？
It was the last film at the cinema，which was to close next day.
After dinner they were to go to a movie.
was/ were to do表示过去曾经计划要做的事，或者过去应当做的事，而且从现在的角度来看已经实现了。如：
I felt nervous because I was soon to leave home for the first time.
I was to play Juliet.
The expedition was to start in a week’s time.
was/ were to have done,表示“本打算”、“本计划”或“本应当”做的事而没有做成或没有发生。如:
I was to have seen him last Wednesday, but he didn’t come.
We were to have been married last year.
2.表示“义务”、“应该” 。(意思接近于should， must， ought to， have to)如：
No one is to leave the building.谁也不得离开这楼房。
You are not to smoke in this room.=You are not supposed to smoke in this room.
You are to be back by 10 o’clock.你必须十点以前回来。
3.表示可能性,相当于can, may如:
Such books are to be found in the library.这样的书在图书馆里就能找到。
Not a single sound was to be heard.一点儿声音也听不到。
Not a soul was to be seen.看不到一个人影。
She was nowhere to be found.哪儿也找不到她。
She was never to see his wife and family again.
5.表示“命运”,将来必然要发生的事,译作“注定……”。如：
He came to power, but he was to play dearly for it: soon he was assassinated.
The worst is still to come.
They said goodbye, little knowing they were never to meet again.
6.用于“if…were to do”，表示虚拟语气。如：
If it were to rain tomorrow, the sports meeting would be put off.如果明天下雨,运动会就会延期。
什么时候不定式表将来
be to do 就表示将来，不同于进行时的是它的计划性更强 例如：He is going to be a doctor.(么得把握滴，表一种愿望)He is to be a doctor。（或者现在从医学院毕业了，马上就去做医生了）参考 一般说来，“be to do”这个结构有两种语法意义，其一是连系动词be+动词不定式做表语，其二是...
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不定式表主动和将来
在这些句子中，不定式“to visit”和“to become”表示的是将来会发生的动作，即将来时的动作。
什么时候不定式用于做表语?
(1)不定式作表语一般表示具体动作，特别是表示将来的动作。 A.不定式作表语。 例如 ：To do two things at a time is to do neither.一次做两件事等于未做。 What I would suggest is to start work at once.我的建议是立刻开始干。 B.如果主语是不定式(表示条件)，表语也是不定式(表示结果)。
to+不定式表目的与表将来区别
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不定式用来表示将来时态时。什么意思举个例子?
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句子中什么时候用不定式?
用法太多，这里写几种。第一，用不定式表将来的目的性行为.第二，用不定式修饰被序数词,最高级,no,any,all所限定的中心词.第三，用不定式修饰抽象名词.(ability,chance,idea,excuse,promise,等等)
将来时的结构
以及计划、安排、打算要做的事。3、be+不定式表将来，按计划或正式安排将发生的事。4、beaboutto+不定式，意为马上做某事,不能和表示明确时间的时间状语连用。5、现在进行时表将来时，现在进行时和短暂性动词连用时可以表将来。6、一般现在时表将来,和短暂性动词连用可以表将来计划或安排好的事。
