چهار کاربرد be to do sth (که اصطلاحی رسمی می باشد)

be to do sth formal  

a) used to talk about arrangements for the future صحبت در مورد برنامه ها در آینده
 Audrey and Jimmy are to be married in June.
 Two men are to appear in court on charges of armed robbery.
 We were to have gone away last week, but I was ill. 

b) used to give an order or to tell someone about a rule دستور دادن یا گوشزد کردن در مورد یک قانون
 You are to wait here in this room until I return.
 All staff are to wear uniforms. 

c) used to say or ask what someone should do or what should happen پرسیدن در مورد اینکه بهتر است چه کار کنیم
 What am I to tell her?
 He is not to be blamed. 

d) used to ask how something can be done پرسیدن در مورد چگونگی انجام کاری
 How are we to get out of the present mess? 

منبع: لانگمن http://www.ldoceonline.com/dictionary/be_1