Probably the Dativus commodi as in Latin and Greek.

+Matt Dinse
I see - that makes sense. Thank you.

Very probably he meant the use of the dative to denote who benefits from the action. This was common in Latin (see the Dativus commodi here: https://en.wikipedia.org/wiki/Dative_case#Latin). I am not sure whether this is explicitly called "dative of advantage"  in English, but it probably is (my grammar when I studied Latin called it a "dativo di vantaggio" in Italian), and Tolkien must have been familiar with it.

The semantic reason behind the use of the dative is also quite clear: who benefits from the action is somehow the "recipient" of the whole action.

I think we can consider the "nin" in the famous "Sí man i yulma nin enquantuva?", as an example of this "dative of advantage", and it is in fact translated as "for me".

The modern term is 'beneficiary'. I actually showed a semantic map of the dative at the Omentielva, comparing natural languages with Quenya.

The difference between recipient (this is what Tolkien most probably means by 'pure dative') and beneficiary is in fact made in English: The former requires 'to', the latter 'for'. On the other hand, English 'to' makes no difference between direction and recipient.

According to the description in GL, the Goldogrin dative has all the three functions. It's good that you mention it, I'll have to include that.

I see. My native Hungarian sort of distinguishes between these; it has a suffix -ért ("for the benefit of", also "for the purpose of") and -nak/-nek ("for, to").

I've been interested in Hungarian for a while; I'll look forward to your interests insights when the papers on Mágol are published.

*edited for typo

I'm happy to provide information here just ask away, +Matt Dinse:) Yes, I am looking forward to that publication - although it has been in preparation for years now I reckon!

Thanks, Tamas; last year sometime I was musing on how some aspects remind me of Quenya, and wondering how one might translate directly Magyar <> Quenya without using English as a middleman.

However, I recently heard something (from Andrew Higgins / Wotan, iirc) about the Taliskan material being next on the list for publication and coming out sometime next year, though I'm not sure about sources, and whether it's substantial enough to need to fit into a PE, or less pages and able to fit into a VT. I think the Mágol papers and analyses would be in VT, but I'll have to browse for references in the currently-published issues to see if it notes anything on the amount of Taliska documents. Maybe someone else would know more off the top of their head.

I don't know. Rumours about a Taliskan grammar were circulating on Elfling many years ago, but I can't recall the details.
Interesting question about translating directly from Magyar to Quenya. In a way, yes, the similarities make it easier, Magyar being an agglutinating language, its logic is close to that of Quenya, and may come more naturally to a Magyar speaker. On the other hand, all our sources, all glosses, categorizations, explanations, definitions etc. are in English, and that inevitably colours our understanding and the way we learn.
For those Hungarians who learn study Quenya from a Hungarian guide, it may be different, but I have no first-hand knowledge of that experience.