how to use "to be to" as a modal verb
Here I could find the answer but it's still unclear to me http://linguapress.com/grammar/to-be.htm

Question

Answer

"to be to" is hard to explain because it can have many different meanings, but in each case, usually only one meaning is correct. These meanings can include:

"I am to"
=I should or I am supposed to
=I must
=I will or I am going to

In the past tense, "I was to", it often means something that should have happened but did not.  "I was (going/supposed) to fly to London this morning, but the weather was too bad."