grammatical number - Verb agreement in "[adjective] and [adjective] [noun]" constructions

Context: I am writing a short text to explain the main differences between Spanish passive sentences with "ser" and "estar", one of which is verb choice: sometimes the adjectival form should be chosen, and sometimes the participle form should. Those are generally the same, but sometimes they differ. The problematic sentence goes like:

Some examples where the participle and the adjectival form differ are: [and a list follows]

In a previous post, it was explained that the noun (form) can be either in singular or plural. Because the noun was a general thing, I decided to go with the singular form, but then a verb followed (differ). Now, my question is:

• Should the verb be conjugated in third person singular (differs) or plural (differ)?

Common sense tells me that it should be a plural (since there are two forms), but "differ" after "form" doesn't sound right (or does it?). I know I could just change form to forms, but I would like to keep it singular. Besides, I really am interested in knowing how the verb should be conjugated in this kind of phrases when the noun is to be kept in singular.

Answer

When you say

where the participle and the adjectival form differ

you've chosen a rhetorical device called prozeugma, which means that preceding phrases (here, "the participle" and "the adjectival") govern a single following word (here, "form"). What this means is

where the participle form and the adjectival form differ

As you've noted, the choice of the number of form belongs to the author, and you've chosen the singular. You could have chosen not to use prozeugma, and instead written the plural forms, making "participle and adjectival" a single compound modifier.

But no matter which choice you make, you still have a compound subject, two things that differ from each other, which makes two differences, one from each point of view. That requires a plural verb, namely differ. It's true that the singular noun form might be slightly jarring next to the plural verb differ, but grammatically it's the plural subject that matters.

It is possible to use the singular differs, but you'll have to rephrase so that the verb is governed by a singular subject:

where the participle form differs from the adjectival form

Of course it's still true the other way around, but grammatically, you speak to only the one difference, so the verb must be singular.