一般說來，“be to do” 這個結構有兩種語法意義，其一是連系動詞be+動詞不定式做表語，其二是be to是一個獨立詞匯單位,具有情態含義,可以把它叫做情態習語(modal idiom)。

一、be +動詞不定式, 不定式做表語,表示主語和表語在概念上是等同的。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 +動詞不定式中的be to用作情態習語, 這時的be to do表示: “計劃”、“安排”、“義務”、“應該”、“可能”、“命運”等。(have to, ought to)。如:
He is to have a holiday. (表示將來)
The committee is to meet today. (表示計劃、安排)
You are to go to the hotel where rooms have already been booked for you.

1. 表示 “將”、“計劃”、“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 表示過去曾經計劃要做的事，或者過去應當做的事，而且從現在的角度來看已經實現了。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”、“本計劃”或“本應當” 做的事而沒有做成或沒有發生。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”、“應該” 。(意思接近于should， must， ought to， have to) 如：
No one is to leave the building. 誰也不得離開這樓房。
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. 你必須十點以前回來。

3. 表示可能性, 相當于can, may 如:
Such books are to be found in the library. 這樣的書在圖書館里就能找到。
Not a single sound was to be heard. 一點兒聲音也聽不到。
Not a soul was to be seen. 看不到一個人影。
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”, 將來必然要發生的事, 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”，表示虛擬語氣。如：
If it were to rain tomorrow, the sports meeting would be put off. 如果明天下雨, 運動會就會延期。

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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