Be  to + infinitive :

1/ To convey orders and instructions:

Ex:

• At the end of the week , all students are to take two exams.

• No one is to leave the building without  my permission.

• You are to stay here till I return.

2/ Official arrangements or plans:

Ex:

• The prime minister is to visit Canada next week.

• A new train line is to be built. (Passive form)

3/Past forms :

A/ Was/were  to + infinitive for things that happened:

Ex:

• He was to travel to Paris. ( He planned the travel  and he did travel.)

B/ Was/were to + perfect infinitive for unfulfilled plans:

Ex:

• The manager was to have made a historic speech last week, but he was taken ill. (He didn't make the speech.)

• The meeting was to have been held last Monday, but it was put off due to unforeseen circumstances.(Passive form)

4/ Be to+ infinitive in if-clauses: used to talk about preconditions for something to happen.

Ex:

• If Ali is to succeed, he needs to study hard.

• If we are to solve the world's pollution problems, we must address environmental issues now.

**** Note:

We don't use be to +infinitive to talk about events that can't be controlled by people:

Ex:

• There are fears that sea level  will rise catastrophically in the next 50 years. ( Not is to be rise......)