I don’t know if this is the one you mean John - there’s a downloadable PDF list of prepositions here http://www.bbc.co.uk/wales/learnwelsh/grammar/index.shtml (you actually posted the same link in this thread in September last year 
)
Also, there’s very handy little book I can recommend called “Pa arddodiad?” by D.Geraint Lewis. It tells you which verbs use which prepositions -http://www.gwales.com/goto/biblio/cy/9781859027646/?lang=EN&tsid=1
yup, very common in certain parts of Wales 
Great thanks Siaron. The one I can’t find at the moment shows which verbs have wrth, i or efo/gyda. Ill look again.
I’m on week ten and reviewing a couple of challenges. I keep coming across this word that sounds like nadd. The sentence is Rhywun nadd dweud wrtha’i… someone who told me… I’ve read back and I can’t figure where we learned this. It even says rhywen ddwedodd for this sentence in Ch 15. It’s come up in other places too and baffled me. I do feel like I’ve learned it. Helpu anyone?
The word you are after is “naeth” (pronounced like ‘nahth’). It’s the preterite (a completed action in the past) tense of “gwneud” (to do or make).
Comparing it to my attempts to write Welsh I hear, I would bet on oedd, because I tend to stick letters from the previous word to the next! 
But I don’t think it works in your example, though. 
Oh, or maybe ddwedodd?
p.s. I’d like to point out that @gruntius is more reliable, he knows way more Welsh than me- it’s just I’ve been repeating these level 1 challenges recently and I just don’t remember any naeth right before dweud in the examples! 
Where is it in the challenges? I’ll have a listen. My gut instinct would also be Naeth as @gruntius has said
Yes, I also think Naeth, generally pronounced Nath. TBH, sone social media users tend to spell it Nath to give a natural feel to their posts.
Is there a way to say “I’m going to go …” would it be dw’n mynd I fynd?
Yup, dwi’n mynd i fynd 
I have a little question that I’ve been meaning to ask for a while now. I’ve been watching Pobol y Cwm and twice now I’ve heard the word “siwrne” but not meaning journey.
Here’s an example:
“Siwrne fydda i 'di cwpla hwn.”
The English translation is listed as “Once I’ve finished this.”
Both times that I’ve heard it, the translation has been “once” and I get that unwaith would refer to “one time” and really not once in the way that it’s being conveyed here.
I’d love to hear any insights about this!
This is a wild guess, so I could be way off track. But could that be siwr na (two words)? It would be really idiomatic, I think, meaning something like “sure that…” ??
Apparently, “Once” is correct as the 2nd definition of Siwrna(i). I found it in “Reading Welsh” by D. Geraint Lewis and Nudd Lewis, published by Gomer. The book claims to be a companion list, but I class it as a dictionary that short cuts across grammatical versions of words.
I’m so glad that I picked it up during a visit to Palas Print.
Hi Sionned - that’s exactly the way that I’ve been thinking of it!
Hi John - Oh you are a superstar! Thanks so much for looking that up for me. (I think that “Reading Welsh” might be the next book that I buy…not that I don’t have enough Welsh books already!).
Yes.
Is sneb a more compact version of does neb?
Yep. Similar to 'sdim = Does dim…
’sneb yn gwybod - Nobody knows.
Right mae’n ddrwg da fi for the slow reply. Challenge 16 with the English starting at 18:52. Listening again definitely the unvoiced -th and not -dd and I guessed before.
Hearing it, I’d say it’s a naeth too even though, once again, I don’t remember ever hearing those sentences before! 
(It happens from time to time with Challenges)
p.s. taking advantage of the question…but is wnaeth the same?