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(tài)含義,可以把它叫做情態(tài)習語(modal idiom)。 下面小編就和大家分享下 be to do的用法。

　　be to do在句子中的成分：

　　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(tài)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(jīng)計劃要做的事，或者過去應當做的事，而且從現(xiàn)在的角度來看已經(jīng)實現(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”、“本計劃”或“本應當” 做的事而沒有做成或沒有發(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”、“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.

　　5. 表示 “命運”, 將來必然要發(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.

　　6. 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。

　　"be to do"的5種用法

　　一、表示“按計劃、安排即將發(fā)生某事或打算做某事”。

　　例：She is to be seen in the lab on Monday.(星期一你準會在實驗室見到她。

　　二、該做或不該做的事情(語氣上接近于should, must, ought to, have to)，表示一種命令、規(guī)勸性語氣。

　　例：You are to go to bed and keep quiet, kids. Our guests are arriving in less than 5 minutes.(孩子們，你們必須上床睡覺，不準吵鬧。我們的客人5分鐘之內(nèi)就要到了。)

　　三、能或不能發(fā)生的事情(接近can, may)

　　例：How am I to pay such a debt?(我怎么可能還得起這么大的一筆債呢?)

　　四、不可避免將要發(fā)生的事情，后來將要發(fā)生的事情。

　　五、用于條件從句“如果……想，設想”(接近if ……want to,或if ……should)