It is used in two tenses - The Present and The Past Indefinite.
It expresses:
Possibility due to previous agreement.
I was to send them this information by fax.
Я должен был отправить им эту информацию по факсу.Possibility due to some rules, laws, regulations.
You are not to speak to the prisoners here, madam.
Здесь не положено (нельзя) разговаривать с заключенными.Possibility due to circumstances with the Passive Infinitive only.
This mineral is to be found in the Urals.
Этот минерал можно найти на Урале.He is not to be found anywhere.
Его нигде нет.
Notes:
The modal verb to be can express wish in adverbial clauses of condition.
If we are to win terrorism the governments of all countries must unit their efforts.
Если мы хотим победить терроризм, правительства всех стран должны объединить свои усилия.The modal verb to be can be used to inform of something, which was a fact of the future for the discussed moment but is a real fact from the point of view of the present moment.
On board of that ship there was a young Scottish gentleman whose name was Lermont. He was to become the great grandfather of the second most famous Russian poet Lermontov.
На борту корабля был молодой шотландский джентльмен, которого звали Лермонт. Он должен был стать прапрадедушкой второго самого знаменитого русского поэта Лермонтова.