依主詞來對 Be 動詞做現在式的詞形變化 + 主要動詞的不定詞
未來的義務，以 to travel 為例
|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?|
在英語的書寫中，我們可以用此句型來論及未來要履行的義務或要求。它的意思跟 must 相似，但有「某事已為我們安排或籌劃」之意，一般並不會用在英語的口語上。
- 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.