依主词来对 Be 动词做现在式的词形变化 + 主要动词的不定词
未来的义务，以 to travel 为例
|I am to travel.||I am not to travel.||Am I to travel?||Am I not to travel?|
|You are to travel.||You are not to travel.||Are you to travel?||Aren't you to travel?|
|He is to travel.||He is not to travel.||Is he to travel?||Isn't he to travel?|
|It is to travel.||It is not to travel.||Is it to travel?||Isn't it to travel?|
|We are to travel.||We are not to travel.||Are we to travel?||Aren't we to travel?|
|They are to travel.||They are not to travel.||Are they to travel?||Aren't they to travel?|
在英语的书写中，我们可以用此句型来论及我们未来要履行的义务或要求。它的意思跟 must 相似，但有「某事已为我们安排或筹划」之意，一般并不会用在英语的口语上。
- You are to leave this room at once, and you are to travel by train to London.
- In London you are to pick up your ticket from Mr Smith, and you are to fly to your destination alone.
- When you arrive, you are to meet our agent, Mr X, who will give you further information.
- You are to destroy this message now.