idioms - "BE to VERB" construction in "you are to blame"

1. The plane is to take off at 9pm
   


2. You are to blame

1 refer to what will happen in the future as the "be + to +verb" construction is normally used for. In 2, "you" is like the object of "blame". Could you explain this usage to me?

Answer

The idiom BE to VERB has three distinct meanings:

1. 
   

   active sense = ‘Be appointed, expected, supposed, obliged to VERB at some point in the future’

   
   

   
   

   I am to go to London tomorrow. ... I am supposed to go to London tomorrow.

   
   

   I was to see the Minister yesterday, but he was called away. ... I had an appointment to see the Minister, but ....

   
   

   The plane is to take off at 9pm. ... The plane is expected to take off at 9pm.

   
   

   
   


2. 
   

   passive sense = ‘Be worthy of being or ‘expected to be VERBed’. This 'passival' infinitive also appears without BE as a postposited modifier. The phrase may also be expressed with an explicit passive, BE to be VERBed, or with for VERBing.

   
   
   

   
   

   This room is to let. ... This room is to be let (=rented).

   
   

   These doughnuts are to share. ... These doughnuts are for sharing.
   I got us doughnuts to share. ... I got doughnuts for us to share.

   
   

   You are to blame. ... You deserve to be blamed.

   
   

   
   

   Compare HAVE to VERB, where to VERB has the same passive sense: ‘We have several treatments to show you’.

   
   


3. 
   

   conditional sense = ‘If [subject] VERBed’. The past form were, without inflection for 3rd person singular, may express a tentative or hypothetical condition; this may be introduced with if, or were may invert with the subject.

   
   
   

   
   

   If he were to win the lottery he might retire. OR
   Were he to win the lottery he might retire. ... If he won the lottery he might retire.