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Certain types of preposition-stranding are considered by some linguists to be "ungrammatical" in English, even though they do not seem remotely strange to me (an English speaker). I'm not talking about prescriptive prohibitions on all preposition stranding, but about allegedly ungrammatical subtypes of stranding.
For example, I recall hearing that the following kind of sentence with "of" is seen as ungrammatical:
He knows what country Brazzaville is the capital of.
I have also seen it claimed that you cannot strand prepositions after an indirect object:
What did you talk to John about?
Am I right that many linguists see these types of constructions as questionable? If so, why are they seen as such, when they seem so completely unexceptional to me and (I would venture to say) a great many of my English-speaking peers?
Thanks for any help