“Be＋to do 不定式”在不同情况下，可以表示很多种不同的语义。下面九种“Be+不定式动词”的语义，稍纵即逝，常常会不多留意，而且也不会时常用到它们，但是只要定睛注意，多少也会领略这些结构的个中意味。
1.表示坚决的命令。例如：
1）This naughty boy is to stay here until the class is over.
2）No one is to enter this building without the permission of the security guard.
2.表示“计划”或“安排的事项”。例如：
3）Betty is to be married soon.
4）An insurance agent is to meet us this afternoon.
3.表示“可能性”。例如：
5）Are you to pay the debt for your wife?
6）The lovely puppy is nowhere to be found.
4.表示“意图”或“打算”。例如：
7）If you are to be there on time, you had better hurry.
8）If all of us are to remain friendly, we must be sincere with one another.
5.表示“应该”。例如：
9）You have done so well. You are to be rewarded.
10）Such nasty questions are to be avoided.
6.表示“不可避免”。例如：
11）Anne was wondering what was to become of her boy friend, Tommy.
12）The murderer was sentenced to life imprisonment and was never to see his family again.
7.表示“假设”。例如：
13）If I were to do it, I would do it well.
14）Even if you were to take a taxi now, you would not be able to arrive at your destinaton on time.
8.有“期待将来”的含意。例如：
15）The actual usefulness of the newly-invented machine is still to be determined.
16）Henry's appointment with the accountancy firm is yet to be confirmed.
9.有“必然会发生”的含意。例如：
17）She did not think too much of her unkind acts. However, they were to be harmful to herself, too.
18）Tom wasn't worried about his head injury, but it was to be troublesome for him later.