Use of be+infinitive in different meanings

Question

I know that be + infinitive is used for order, instruction and plan but some of the example sentences are confusing to me they are:

1. I'm to go now. ( what's the meaning of this sentence ?)

2. Mr johns was to speak at the meeting last monday. ( if this sentence is about past plan, was the plan fulfilled ? Or what's the meanings of this sentence ?)

3. You are to be on time.(what kind of meaning this sentence implies?)

4. Am I to believe what they say ? ( what kind of meaning this sentence conveys ?) Now, if the meaning of those sentences is similar to 'supposed to', why be+infini' is used instead of supposed to' ? Is 'supposed to' is the same as 'should' or what? I'm expecting understandable answer. Thank you in advance.

Answer 1

Be + to-infinitive is used to express:

1. Official arrangements or plans:

We are to meet at his office at six.

The Prime Minister is to visit Poland next week.

2. Orders and obligations:

At the end of the course, all students are to take a written exam.

You are to do your homework before you go to the cinema.

3. Things that should be done:

What am I to do?

You are to stay here until I send for you.

4. A possible aim when saying what should be done to reach it:

If you are to work here for more than three months, you must have a residence permit.

5. What is fated to happen:

I met a girl who was to become a queen.

6. Pre-conditions:

If we are to get there on time we had better hurry.

7. Prohibition (in the negative):

You are not to do that again.

In your examples # 1 is a prohibition, # 2 is an order, # 3 is what should be done, # 4 is a fulfilled plan (otherwise it were to be "to have spoken"), # 5 is an order, and # 6 is asking about whether something should be or is obligatory to be done.

Regarding obligations, the use of "must" expresses the strong ones and usually means that some personal circumstance makes the obligation necessary, and for this reason the speaker almost certainly agrees with the obligation, whereas to be + to-infinitive shows that the obligation is/has to be just put up with no matter what the speaker thinks about it.

You are to return to the regiment by 10 p.m. (a comander's order)

I was to execute the deserter and I did it.( I was ordered to, and orders are peromptory)

He is your brother. You must help him. (it's your moral obligation).

I must stop him before it's too late. (I feel it must be done. Why not by me?)

The source: Michael Swan, Practical English Usage, Third Edition, Oxford University Press, ISBN-13: 9780194420969, p.80, para 91

Answer 2

In most cases, you can replace I'm to with I'm supposed to. In some, it will have the stronger sense of an obligation as you noticed in 1. and 2.

3. I'm to go now.

   I'm supposed to go now / I should be going now.

In 4.:

4. Mr Johns was to speak at the meeting last monday.

   Mr Johns was supposed to speak at the meeting last monday.

Which, in a sense, implies that he didn't.

Since you added 2 sentences, same story:

5. You are to be on time.

   You are supposed to be on time.

Which here, sounds more like a judgement, a scolding from your boss because you were late. And:

6. Am I to believe what they say ?

   Am I supposed to believe what they say ?

The context will help you guess if it is a strict obligation or not, as you already did for the first two sentences.