I came recently across a sentence that goes similar to this: "Natürliche Personen, die Mitglied sind, gelten als fördernde Mitglieder."
In my understating I thought it should be "die Mitglieder sind", but most of my colleagues said it should be singular without knowing why. Also AI tools are giving both as correct.
Would please someone explain, what is correct and why? what is the name of this rule or case?
There is a slight difference in the grammar. It is a little tricky, because the reference is different, but the referent is the same in both cases:
Personen, die Mitglieder sind
refers to groups of people, namingly to all persons who are a member, while
Personen, die Mitglied sind
refers to any person who is a member, i.e. to each single person. English expresses this difference with the quantifiers any and all, in German you use jede vs. alle. In the quoted sentence, there is no logical difference.
This is a copula phrase with a plural subject. There's no rule about the number of the predicative in this case. It may be either singular or plural.
Die fünf Männer sind Mitglied des Vereins.
Die fünf Männer sind Mitglieder des Vereins.
You can see that in your example because gelten als is another copula verb, and it has a plural predicative. You could also write
Natürliche Personen, die Mitglieder sind, gelten als förderndes Mitglied.
or the other two permutations.
Sometimes it matters because of semantics though:
Die fünf Männer sind Verwandte.
It makes no sense to put the singular Verwandter in here because it takes more than one Verwandter if you don't mention to whom a single person is a relative specifically.
