1, 词组“be to + do”一般用于官方或正式的安排与计划、正式的命令、还可以表示“应该”、“必须”等。
- The Prime Minister is to make a further visit to Devon next week.
- We are to receive a pay rise in line with inflation in September.
- I don’t mind her going to Ruth’s party but she’s not to be back late.
- You are not to leave this house without my permission. Is that clear?
2, 词组“be to + do”也经常用于报纸、电视报道等，表示预计将来极有可能发生的事。
- A man is to appear in court later this morning charged with the murder of the footballer, Darren Gough.
- The Ministry of Trade and Industry has announced that it is to move three thousand jobs out of the capital and re-locate them in the North East.
- Work is to begin this week on the new bridge across the Nile north of Aswan.
3, 词组“be to + do”在if条件句中用来说明某事发生的先决条件。
- If we are to catch that train, we shall have to leave now.
- If I were to increase my offer from five hundred to five hundred and fifty pounds, would you be interested in selling me your car then?
- If we are to solve the world’s pollution problems, we must address environmental issues now.
4，“be to + 不定式的被动形式”表示指示，经常可以在用药说明或其它官方通告中看到。
- To be taken three times a day after meals.
- These benches are not to be removed from the changing rooms.
- No food of any kind is to be taken into the examination room.
5, 这种词组结构经常用于讨论现在或将来的安排，而且经常用于现在或过去时态。但是，“be to + 不定式的完成形态”有时用来表示本应该发生但实际没发生的事。
- He was to have appeared in the West End show but broke his collar bone during rehearsals.
- They were to have picked strawberries this morning, but the torrential overnight rain made the field too muddy.
- Sammy was to have married Sarah but then Jamie came along and the engagement ring he had given her was returned.