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When I read an essay or novel which is written in English language

I usually come across sentences that is somehow made me puzzled

I always wonder what type of grammar tenses are they?

And when and how should them be used in conversational chat or in writing essay ?

I'm going to write them down :

-The queen called her huntsman before her. " you are to take Snow White
into the forest," she said.

-I was to call you yesterday but I could not.

-You are to find out who I am.

Subject + Be verb + to + infinitive

what is these sentences grammatically called ?
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Quirk et al. call it a modal idiom "expressing futurity, with varied connotations of 'compulsion', 'plan', 'destiny' according to context. In the past, was to and were to express futurity from the standpoint of past time orientation."
Comments  
Students: We have free audio pronunciation exercises.
AnonymousI was to call you yesterday but I could not.
-You are to find out who I am.
This type of sentences typically have an obligatory and commanding connotation.

Every employee is to follow company codes of conduct. He is not to use company resources for personal use.
how could i use it correctly and when ?

what is it called in grammatical term ?
I have already told you that it is a modal idiom. It is a rather formal structure used, as Quirk said, for future actions, often with the feeling of compulsion, plan or destiny.

you are to take Snow White into the forest -- command for the future

I was to call you yesterday but I could not.-- a plan (future-in-the-past)

You are to find out who I am. -- command for the future

I am to be in Albuquerque on the 16th -- future schedule

etc.
Students: Are you brave enough to let our tutors analyse your pronunciation?