"Before the petrol finished, I went to the garage."
Translation:Cyn i'r petrol orffen, es i i'r garej.
modur + tŷ > modurdy (literally "motorhouse")
The second word in a compound gets the soft mutation, and the circumflex is lost. I don't think it's anywhere near as common as garej though.
It appears in the Geiriadur Prifysgol Cymru: http://geiriadur.ac.uk/gpc/gpc.html?modurdy
I'd never heard of modurdy before this translation came up. It's certainly not in the "Oxford Pocket Modern Welsh Dictionary" by Gareth King which is used by many learners, so I'd agree that it seems to be archaic. I do, however, like how it came about (Thanks Balbhan!), and do actually like it better than the English import, however, in trying to learn to speak the language, I'll stick to the latter as I guess I'll be understood better (and that's my aim!)
I did guess that that was how it was put together but I personally have never seen or heard that before. Did the course teach modurdy?
I looked up the entry and it seems that it was first used in 1934 so I would guess that it could be considered archaic by now.
I've only seen modurdy in multiple choice questions. Words reported by users as correct can appear in multiple choice - even if they aren't taught. I expect that's what happened.