Difference between Be+infinitive and must

Question

I know that be + infinitive is used for ordering, giving instruction, and planning, but some of the example sentences are confusing to me. They are:

1. Nobody is to open the door. (order)

2. You are to do your homework before you go out. (order)

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

4. Mr. Johns was to speak at the meeting last Monday. (Is this sentence about a past plan or obligation?)

5. You are to be on time. (Is this an order?)

6. Am I to believe what they say? (Is this sentence an obligation?)

I'm confused because it's said that be + infinitive is used in giving an order, but I think the meaning is the same to that of 'must'? So, how is 'be + infinitive' different from the meaning of 'must' when giving an order or when there is an obligation? Could you please provide an understandable answer? Thank you in advance.

Answer 1

The be + infinitive structure is used in several ways. The main ones are:

The president is to visit Sudan next week - official plans

You are to change the towels every day - order or instruction (must)

You are not to smoke inside the hospital - permission (not allowed)

Am I to believe this nonsense? - supposed to

Looking at the sentences you suggested: 1 could be orders or permission- more context is required to decide. 2 and 5 are orders. 3 might be an order or an official plan- more context is required to decide. 4 is both an official plan and supposed to. 6 is supposed to.

In sentences 2 and 5, you could replace to + infinitive with must, and it would have exactly the same meaning.

In sentences 1 and 3, if the intended meaning is orders, the same applies. If 1 is permission, you can replace it with may not, and in 3 if it is an official plan you can replace it with scheduled to go.

If you want to make your statement completely unambiguous, it would be better to avoid be + infinitive, and instead use the appropriate alternative form- must, may, scheduled to, supposed to.

For an English learner, it is important to be able to recognize and understand this construction in case you read or hear it from other people, but I would recommend sticking to the alternative forms when writing or speaking.

Answer 2

Your thinking that "be+infinitive" is the same as "must" is not quite correct although I can see how you might think that.

For simplicity, you might want to think of "must" as a requirement (obligation), whereas "be+infinitive" is an expectation or directive (order), something you are "supposed" to do (but if you do not there may or may not be consequences). With "must" there are always consequences.

Nobody must open the door under any circumstances.
You are required not to open the door

Nobody is to open the door except in an emergency.
Nobody is supposed to open the door

"Must" encompasses the extreme condition of "be+infinitive", and so in the extreme both are interchangeable. If you are equating "order" with "obligation" (in the military sense) then you are only dealing in the extreme and they are equivalent.

Answer 3

In my American English, most of these sentences could be rearranged to use "must" or, in some cases, "need to." To my ear, using be + infinitive sounds fairly old fashioned and/or British.

For example, I could rearrange your first sentence to say "Nobody must open the door." This sentence cannot use "need to" because saying "Nobody needs to open the door" implies that it is not necessary for anyone to open it, and not that nobody should.

Your second sentence can use either "must" or "need to" with no rearranging.

The third sentence sounds very outdated to me. It would be better written as "I need to go now" or "I must go now." This sentence implies that, for one reason or another, the writer must leave.

The fourth sentence cannot be rearranged the same way as the rest, but definitely seems outdated. This sentence seems to imply that Mr. Johns did plan to speak in the past, but did not. While the sentence does not say that he did not, the structure "was to" gives the impression that it did not happen. I would write this sentence as "Mr. Johns was supposed to speak at the meeting last Monday." In most cases I would follow this with "but..." and a reason why he did not.

Your fifth sentence is an order. The writer is saying that you must be on time. I would swap "are to" with "need to" or, if I were being more strict, "must."

The last sentence is the only one that does not sound as outdated to me. This sentence cannot be changed like the others. The writer of this sentence seems to be asking whether or not "they" are trustworthy. In any case, if I were to say this myself, I would write, "Should I believe what they say?"

Answer 4

I think that, generally speaking, be+infinitive and must are largely interchangeable and others here have posted about the subtle difference between order and instruction that you can see sometimes.

There is one usage though where the difference is quite marked. Take the following example:

You are to read The Wasteland by TS Eliot

That's a clear instruction. The sort of thing that you would see in a homework assignment. However:

You must read The Wasteland by TS Eliot

Now, that's far more ambiguous. It could be an instruction as in the previous example but it could also just be a recommendation, carrying no more weight than "you must watch the latest episode of Game of Thrones"

Answer 5

Here's an example where must is preferable to be to:

A: Great, my car won't start.

B: Well, now you must walk to work.

To be to X generally implies someone is requiring or commanding you to do X. Must can be used for this purpose as well.

Since you are in trouble, you are to walk instead of taking the car to school for the rest of this week.

Since you are in trouble, you must walk instead of taking the car to school for the rest of this week.

Must can be a little more polite since it can be used to indicate a situation is obligating you as opposed to a person obligating you to do something. Example:

Hello team. Today we must make sure we verify all reports.

versus

Today we are to make sure we verify all reports.

Answer 6

Must is a general-purpose verb that can be used to express any kind of obligation, requirement, or necessity:

We must eat to live.

I must pass a lifeguarding test for my summer job.

I must remember to thank her for that thoughtful gift.

I must change the bandage every six hours.

whereas to be + infinitive expresses a narrower range of arbitrary obligations, namely, those which have been imposed by an authority, or by the dictates of a prearranged schedule or itinerary, or by some rule. So we can substitute to be + infinitive for must in such circumstances.

^{not grammatical} We are to eat to live.

I am to pass a lifeguarding test for my summer job.

I am to remember to thank her [but only if instructed not to forget].

I am to change the bandage every six hours.