什么时候不定式表将来

be to do 就表示将来，不同于进行时的是它的计划性更强

例如：
He is going to be a doctor.(么得把握滴，表一种愿望)

He is to be a doctor。（或者现在从医学院毕业了，马上就去做医生了）

参考
一般说来，“be to do”这个结构有两种语法意义，其一是连系动词be+动词不定式做表语，其二是be to是一个独立词汇单位,具有情态含义,可以把它叫做情态习语(modal idiom)。

一、be +动词不定式,不定式做表语,表示主语和表语在概念上是等同的。如：

The problem is to find a solution.

His plan is to clean the room.

My wish is to be a doctor.

二、be to +动词不定式中的be to用作情态习语,这时的be to do表示: “计划”、“安排”、“义务”、“应该”、“可能”、“命运”等。(have to, ought to)。如:

He is to have a holiday. (表示将来)

The committee is to meet today. (表示计划、安排)

You are to go to the hotel where rooms have already been booked for you.

1.表示“将”、“计划”、“安排”。(意思接近于be going to)如：

Their daughter is to get married soon.

Who is to question him？

It was the last film at the cinema，which was to close next day.

After dinner they were to go to a movi