Like many people here probably, I’ve always loved the hymn tune “Cwm Rhondda”.
When I started learning Welsh, I thought it would be a nice idea to try to learn the Welsh words.
That’s when I discovered the situation wasn’t 100% straightforward. What I didn’t know then, but probably people here do know, is that the Welsh words normally sung to the tune of “Cwm Rhondda” are from a completely different hymn by a different author.
The English words start “Guide me o thou Great Jehovah” (or “Redeemer”) and have the well-known rousing chorus “Bread of Heaven”. There is a Welsh version of that hymn (although the translation is far from literal - and I think the Welsh version came first), although apparently it’s normally sung to a different tune.
It’s all explained quite well in the Wikipedia page:
However, today, by chance, I found this quite interesting Youtube video about it:
As well as some history, it also includes English and Welsh versions, with bi-lingual subtitles.