Why is “that I” 'bo fi" except when before “well i mi”?

I think it’s ‘one of those things’ where two different concepts in Welsh are translated in English using the same words. The phrase ‘mae’n well i mi i’w wneud e’ actually literally means ‘it’s better to me to do it’. But that’s not how English expresses that concept - it uses a ‘that’ structure instead.

Or is the question that the “fi” is missing?

bo fi eisiau = that I want

bo well i fi (/ mi) wneud rhywbeth = that it is better for me to do something

(The English may be phrased as “that I had better…” although the translation is actually of “it”.)

Hope that makes sense, sorry if not!

Ah… I might have misunderstood the question. In which case netmouse is probably giving a better answer!