Cambridge Grammar of English Hardback with cd-rom a comprehensive Guide

What is desirable

Be to

is used to refer to necessary or desired states of affairs:

Children need access to a healthy diet if they 

are to achieve their full potential,

whether physically or mentally. 

(… if it is desirable/considered necessary that they achieve, or if it is the goal

that they achieve …)

[university literature seminar]

The text is fiction, and 

is to be read as such.

Hypothetical future

The subjunctive form were to (and its more informal alternative was to, used with

singular subjects) can be used to refer to a hypothetical future:

I’d hate it if anything 

were to happen to them.

(or: I’d hate it if anything happened to them/if anything should happen to

them.)

I suppose if I

was to be honest, I’d have to say no to it.

(more informal)