english question

先說
be ＋to-infinitive（動詞不定式）
其實有其特別的用法
１）可以用來表示計劃與安排，特別是官方安排（可以看為一種非常肯定的將來式，比will, be going to, will be v-ing更肯定）
如：Obama is to visit China next month.
２）用來指注定要發生的事
如：He was to regret that decision for the rest of his life.
３）用於命令，如：
You are to do your homework before you watch TV.
４）也用於if的從句之中，假於主句是先決條件
如：
If we are to get there by lunchtime we had better hurry.
(we had better hurry是先決條件，以達到能準時到達的結果）
你的句子就是第４頪
1)if we carry out these mission是普通的if從句，其後可帶出結果
如：if we carry out thest mission, we will die.
而
2)if we are to carry out these missions是第４類的用法，所以主句是說明先決條件
如：if we are to carry out these missions, we have to gear ourselves up

希望幫到你！