Varying modal verbs in counterfactual ("subjunctive") conditionals

Question

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

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

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

"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."

"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?

Anton · Answer

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

Varying modal verbs in counterfactual ("subjunctive") conditionals

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