Hello everyone!
Begginer here.
I was wondering if native speakers could tell me if the phrase “rhaid rhoi i’r hud i weithio” makes any sense at all.
Thanks a lot for any help 
sort of - I think it’s trying to say “must put the magic to work”, but grammatically ‘rhaid’ needs to be followed by a noun or pronoun, not a verb (rhoi), and “to work” in this sense would be “ar waith”
So if you wanted to say “I must put the magic to work”, I’d suggest “(mae) rhaid I mi roi’r hud ar waith” - I’ve put the ‘mae’ in brackets because it should be there technically but is often dropped in speech.
Hope that helps.
PS - but it just goes to show, even if it’s not completely correct, it can be understood. Mistakes are good, and attempting sentence-building even with mistakes is a huge step forward! 
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I like Siaron’s suggestion of “ar waith” here. I’d also add that “rhaid rhoi” can be grammatical too - if you imagine it as “mae’n rhaid rhoi…” with the “mae’n” elided. “Rhaid rhoi” is kind of equivalent to “rhaid i [rywun] roi”, except you avoid having to specify who the “rhywun” is 
Thanks a lot, Siaron