一般说来，“betodo”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beto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（modalidiom）。
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。
二、beto+动词不定式中的beto用作情态习语，这时的betodo表示：“计划”、“安排”、“义务”、“应该”、“可能”、“命运”等。（haveto，oughtto）。
英语动词不定式
对于英语来说接不定式的单词多为“打算”“计划”“希望”“将要做某事”等表示未来（未做的事）的动词。同样与不定式表示不同,现在分词表示正在进行，一般性或经常性发生的行为。
(1)表示将来的动作
(2)与被修饰词之间有动宾关系，如是不及物动词，则需加介词
(3)与被修饰词之间有动宾关系，同时与句中其它词之间又有逻辑上的主谓关系时，尽管有被动含义，却仍 用主动语态；如只有动宾关系，而无逻辑上的主谓关系，则需用被动语态。
以上内容参考：百度百科-动词不定式
温馨提示：答案为网友推荐，仅供参考
第1个回答 2013-07-23
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。本回答被网友采纳
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。本回答被网友采纳
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