The construction be to + infinitive is unusual in that there is no intervening word between be and to as in expressions like be about/apt/(un)likely/bound/liable/ sure/certain to. Furthermore, except in referring to the construction as such, it does not occur in infinitive form:
He may be about to leave
He may be likely to leave
He may be apt to leave
…etc.
but
*He may be to leave
The construction has three basic meanings: 'obligation', 'possibility', and 'future'. In most cases, the modal verbs must (or should), can and will can be substituted for be to, with no change in meaning, e.g.:
obligation
You are to be home no later than midnight.
= You must be home no later than midnight.
Mother says I am not to talk to you.
= Mother says I musn't talk to you.
Such men are to be pitied.
Such men should be pitied.
It is to be regretted that so many traditional values have been discarded.
= It should be regretted that so many traditionalvalues have been discarded.
possibility
The custodian was not to be found anywhere.
= The custodian could not be found anywhere
Deer are to be seen wherever humans are scarce
= Deer can be seen wherever humans are scarce
future
What is to become of me?
= What will become of me?
The meeting is to take place tomorrow.
= The meeting will take place tomorrow.
They parted company, but they were to see each other later that evening.
= They parted company, but they would see each other later that evening.
There is also a fourth meaning of be to + infinitive (cf. Edmondson et al. 1977:275), which is close to that of be supposed to, and which is found in rhetorical questions and followed by verbs of inert cognition, and only in the past tense, as in
How was I to know that?
= How was I supposed to know that?
For extensive examples of the use of this structure by German learners, as discussed in the following three sections, click here.