一般說(shuō)來(lái)，“be to do” 這個(gè)結(jié)構(gòu)有兩種語(yǔ)法意義，其一是連系動(dòng)詞be+動(dòng)詞不定式做表語(yǔ)，其二是be to是一個(gè)獨(dú)立詞匯單位,具有情態(tài)含義,可以把它叫做情態(tài)習(xí)語(yǔ)(modal idiom)。

一、be +動(dòng)詞不定式, 不定式做表語(yǔ),表示主語(yǔ)和表語(yǔ)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 +動(dòng)詞不定式中的be to用作情態(tài)習(xí)語(yǔ), 這時(shí)的be to do表示: “計(jì)劃”、“安排”、“義務(wù)”、“應(yīng)該”、“可能”、“命運(yùn)”等。(have to, ought to)。如:
He is to have a holiday. (表示將來(lái))
The committee is to meet today. (表示計(jì)劃、安排)
You are to go to the hotel where rooms have already been booked for you.

1. 表示 “將”、“計(jì)劃”、“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 表示過(guò)去曾經(jīng)計(jì)劃要做的事，或者過(guò)去應(yīng)當(dāng)做的事，而且從現(xiàn)在的角度來(lái)看已經(jīng)實(shí)現(xiàn)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”、“本計(jì)劃”或“本應(yīng)當(dāng)” 做的事而沒(méi)有做成或沒(méi)有發(fā)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 “義務(wù)”、“應(yīng)該” 。(意思接近于should， must， ought to， have to) 如：
No one is to leave the building. 誰(shuí)也不得離開(kāi)這樓房。
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. 你必須十點(diǎn)以前回來(lái)。

3. 表示可能性, 相當(dāng)于can, may 如:
Such books are to be found in the library. 這樣的書(shū)在圖書(shū)館里就能找到。
Not a single sound was to be heard. 一點(diǎn)兒聲音也聽(tīng)不到。
Not a soul was to be seen. 看不到一個(gè)人影。
She was nowhere to be found. 哪兒也找不到她。
She was never to see his wife and family again.

4. 表示 “命運(yùn)”, 將來(lái)必然要發(fā)生的事, 譯作 “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.

5. 用于“if…were to do”，表示虛擬語(yǔ)氣。如：
If it were to rain tomorrow, the sports meeting would be put off. 如果明天下雨, 運(yùn)動(dòng)會(huì)就會(huì)延期。

Exercises:
1.This film _____ this Sunday.
A. is going to show B. is showing C. is to be show D. will have been show

2.The students were told that they ____ at the school gate at 2:00 the following afternoon.
A. met B. will meet C. were to meet D. were met

3. The house ______ ready today but as there has been a builders’ strike it is still only half finished.
A. is B. was to be C. was D. was to have been

4.In such dry weather, the flowers will have to be watered if they _________.
A. have survived B. are to survive C. would survive D. will survive

5.How _______ I ______ what has become of him?
A. am; to know B. am; knowing C. was; to know D. will; know

6. ______ he ________ tomorrow, I would go to meet him at the station.
A. Were; to come B. Was; coming C. Did; come D. Would; come

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