将来的义务
型态
当我们在谈有关将来的义务时，可以用两个要素来构成正式的句型：
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
| 肯定句 | 否定句 | 疑问句 | 否定疑问句 |
|---|---|---|---|
| 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? |
作用
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。
举例
- 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.