1. Be to+不定式
“be to+不定式”用于表示正式的安排、说明以及发出命令等。新闻报道中常用这种结构描述将来的事情。
“be to+不定式”只用于表示人们能够加以控制的事情：
We don’t know where the meteorite is going to land.（不用…the meteorite is to land.）
I suppose we will all die eventually.（不用…we are all to die…）
“be to+不定式”常用于if从句中，来说明只有当主句中的某种情况先发生时，if从句中的另外一件事情才会发生：
If humans are to survive as a species,we must address environmental issues now.
The law needs to be revised if justice is to be done.（被动形式）
请比较在IF从句中用“be to+不定式”和用一般现在时表示将来的用法：
Jones needs to improve his technique if he is to win gold at the next Olympics.和
Jones has said that he will retire from athletic if he wins gold at the next Olympics.
2. 将来完成时
将来完成时用来说明某事在将来某个特定的时间会结束、完成或达到：
Let’s hope the volcanic eruption will have finished before we arrive on the island.
By the time you get home I will have cleaned the house from top to bottom.
表示将来不确定的情况时，可以用其他情态动词代替will：
“be to+不定式”用于表示正式的安排、说明以及发出命令等。新闻报道中常用这种结构描述将来的事情。
“be to+不定式”只用于表示人们能够加以控制的事情：
We don’t know where the meteorite is going to land.（不用…the meteorite is to land.）
I suppose we will all die eventually.（不用…we are all to die…）
“be to+不定式”常用于if从句中，来说明只有当主句中的某种情况先发生时，if从句中的另外一件事情才会发生：
If humans are to survive as a species,we must address environmental issues now.
The law needs to be revised if justice is to be done.（被动形式）
请比较在IF从句中用“be to+不定式”和用一般现在时表示将来的用法：
Jones needs to improve his technique if he is to win gold at the next Olympics.和
Jones has said that he will retire from athletic if he wins gold at the next Olympics.
2. 将来完成时
将来完成时用来说明某事在将来某个特定的时间会结束、完成或达到：
Let’s hope the volcanic eruption will have finished before we arrive on the island.
By the time you get home I will have cleaned the house from top to bottom.
表示将来不确定的情况时，可以用其他情态动词代替will：