1）be to do:
(1)表示事先商定、安排或準(zhǔn)備要做的事情.如：
The students are to meet at the school gate tomorrow.明天學(xué)生們將在學(xué)校大門(mén)口集會(huì).
2）表示可能性,必要、責(zé)任、義務(wù)、禁止等.如：
Her necklace was not to(couldn't)be found.她的項(xiàng)鏈找不到了.
2) be about to do:
1) 表示即將發(fā)生的動(dòng)作,在時(shí)間上指最近的將來(lái).如：
We are about to start.我們就要出發(fā)了.
The new school year is about to begin.新學(xué)年開(kāi)學(xué)在即.
2) 在含有be about to do的句子中,不能再加時(shí)間狀語(yǔ).如：
Wrong:The medical team is about to start immediately.
Right:The medical team is about to start.醫(yī)療隊(duì)就要出發(fā)了.
附：be to do 用法的詳細(xì)講
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.
It was the last film at the cinema,which was to close next day.
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.
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).
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.
They said goodbye,little knowing they were never to meet again.