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I came across a sentence recently:

If you give Angelo the news now it must upset him, so tell him later on.

A native speaker said to me that we can't use "must" here, but unfortunately he was not able to explain why. I wonder if there is anyone who could tell me why we can't use "must" in this sentence.

Perhaps I could say with "will":

If you give Angelo the news now it will upset him, so tell him later on.

NB I know that "could" fits better here, but I would like to know why I can't say "must".

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    The cited usage is syntactically valid, and does convey the meaning you want - it's just not idiomatic. The standard way of indicating "emphatic certainty" in such contexts is: If you give Angelo the news now it's bound to upset him, so tell him later on. Well, that's "standard" in informal conversational contexts. In formal contexts you might consider ...it is certain to... or ...it will surely...
    – FumbleFingers
    Commented Nov 8 at 17:48
  • @FumbleFingers This: If you give Angelo the news now it must upset him. is not idiomatic and also not grammatical.
    – Lambie
    Commented Nov 8 at 19:40
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    @Lambie: Rubbish. It's perfectly grammatical, just no longer idiomatic. Because there are so many false positives, it's not easy to find examples from a couple of centuries ago, but they certainly exist. And the relevant "rules of grammar" haven't changed - just the idiomacy. ... if you do , it must cost you remorse and sorrow
    – FumbleFingers
    Commented Nov 8 at 19:57
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    You're just confusing the issue by switching to Present Perfect, where there's no conditional element.
    – FumbleFingers
    Commented Nov 8 at 20:05
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    That usage of "must" sounds very 19th Century. The sort of phrasing you might find in a Brontë novel.
    – OrangeDog
    Commented Nov 9 at 23:01

2 Answers 2

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I agree with the person you talked to that must sounds odd here. If we look at the definition of must:

  1. be commanded or requested to (e.g., you must stop.)
  2. be compelled by physical necessity to (e.g., people must eat.)
  3. be obliged to (e.g., I must say you're looking well.)
  4. be logically inferred to (e.g., it must be time.)

Merriam Webster

In your first sentence, saying:

If you give Angelo the news now it must upset him.

This usage of must signals that Angelo is compelled or pressured to be upset. That doesn't make much sense.

Will

Yes, this sounds correct.

If you tell Angelo the news now it will upset him.

Unlike could, will means that you are certain. You know what will happen. Could on the other hand means that there are multiple possibilities.

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    I think the definition that best fits that use is #4 -- you infer that he will be upset.
    – Barmar
    Commented Nov 8 at 17:27
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    It's more common to use that after the fact: "You must be upset about that news."
    – Barmar
    Commented Nov 8 at 17:28
  • @Friendly Racoon thank you so much for your detailed answer.
    – Ola
    Commented Nov 8 at 17:41
  • Thank you for your comment @Barmar. It is exactly in that meaning [#4 logical deduction] that I thought "must" was used in this sentence. But reading your answer, I understand that we can't use it here in this sense, because, as you have put it, "It's more common to use that after the fact". I'm afraid I haven't got your point here. Could you expand on it please?
    – Ola
    Commented Nov 8 at 17:42
  • Must as logical deduction: "Must can be used to express the deduction or conclusion that something is certain or highly probable: it is normal or logical, there are excellent reasons for believing it, or it is the only possible explanation for what is happening" (Swan, Practical English Usage, 2016).
    – Ola
    Commented Nov 8 at 17:43
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I. Using must and will for certainty:

OP's query on modal verbs:

  1. If you give Angelo the news now, it must upset him, so tell him later on.
  2. If you give Angelo the news now, it will upset him, so tell him later on.

"Will" is better than "must" here. Let’s explore why:

To express probability or certainty, modal verbs vary in strength:

II. Modal verbs used to show probability/ possibility..

Ref Adventures with Grammar and Composition, Beena Sugatham. Oxford Printing Press. .

  • We can also use may or might to show probability or possibility.
  • May indicates a good possibility and might, a weak possibility though there is very little difference.
  • Can/ could expresses still weaker possibility than might.
  • Must and will can also be used to indicate certainty or likelihood.
  • Must expresses a logical certainty and is used in a context stronger than may.
  • Will is used when we are one hundred per cent sure.

Must shows a logical certainty, often based on current evidence or deduction. For instance:

  • Angelo was close to his wife, so the news must upset him.

Will implies near-absolute certainty about future events or reactions.
In this sentence, because we’re predicting a future reaction to the news, will is the natural choice.
Using could shows very less certainty.

Modal Verbs

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