It's a different kind of "must". When you mean "you must" almost like a command you use Rhaid i, but when you mean "you must" for an assumption then it's Rhaid bod, e.g.
Rhaid i Siân fynd "Siân must go / Siân has to go"
Rhaid bod Siân yn mynd "Siân must be going" (I assume/see)
Rhaid i ti weithio "You must work / You have to work"
Rhaid dy fod ti'n gweithio "You must be working" (I assume/see)
So "You must mean John S, not John H" is Rhaid dy fod ti'n golygu/meddwl John S, nid John H.
An interesting question thanks for raising it.
Meddwl can also be translated as 'to mean' in some specific more complex constructions eg:-
Beth dych chi ei feddwl wrth hynny? - What do you mean by that (lit:- what are you thinking it by that)
This example can only be translated as 'You must think/You have to think'
Thanks. When I did my linguistics course (albeit a few years ago) we were taught not to trust some of the theories posited by the 19th cent. theories of PIE.
Mind you I suppose it can be interesting fo some. But it won't help me get my head around the vagries of rhaid i/bedidio a.