There are some situations in which one would like to vary the modal verb in counterfactual conditionals, but it seems to be incorrect.

  1. "If things were otherwise, she would keep her promise." This seems like perfectly ordinary usage.

  2. "If things were otherwise, she might keep her promise." This also sounds fine.

  3. "If things were otherwise, she must keep her promise." This sounds wrong. But there is an obvious alternative: "If things were otherwise, she would have to keep her promise."

  4. "If things were otherwise, she should keep her promise." This also sounds kind of bad. But I can't think of an alternative way to convey this thought without bending over backwards. (Maybe "If things were otherwise, she ought to keep her promise," sounds better, but the issue seems to be the same. And "ought" often sounds too formal to my ear.)

My specific question is: Is sentence 4 necessarily incorrect? And if so, is there a better way to convey its thought?

More generally: is it ok to vary the modal verbs in counterfactual conditionals, as in examples 3 and 4?

  • For (4), you can use she would be obligated to keep her promise.. – Peter Shor Jul 16 at 13:15

Briefly, and without repeating commonplace definitions of would and should.

As a consequence of things being otherwise:

She would = the promise is kept

She might = there is a possibility that the promise is kept

She must = tries to use must in subjunctive form, which is not appropriate. See Can I use the word "must" in subjunctive mood?

She would have to = she is compelled to keep it

She should = she is obliged to keep it but there is a possibility she does not

  • Thank you for the reply, but this doesn't clearly answer either of my questions. "tries to use must in subjunctive form, which is not appropriate" comes close to answering, but the link you shared seems to be about an importantly disanalogous grammatical situation. – Rick Feb 16 at 18:03

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