Reading Gareth King’s “Working Welsh” I note that if ‘who’ is used in a question then PWY is used. If Who is not part of a question then instead of Pwy, “a” or “sy” or “sydd” is used. Now, on Duolingo I am seeing many apparent inconsistencies with his advice. For examples:
Who is coming? = PWY SY’N DOD?
Who is going? = PWY SY’N MYND
Who is playing? = PWY SY’N CHWARAE?
Who wants a drink? = SYDD AM DDIOD?
Who wants to play golf tomorrow? = SYDD YN MOYN CHWARWAE GOLFF YFORY?
Who can I ask? = PWY ALLA I OFYN?
Who will be playing golf tomorrow? = A FYDD YN CHWARAE GOFF YFORY
I’m a bit confused as all of the above are questions. Is there a simple rule when to use PWY or not?
These are not inconsistencies - as well as looking at ‘Pwy’ in the book, take a look at ‘Sy, Sydd’.
If it’s a question you use pwy as the ‘who’ and because the ‘who’, as well as being the question, is also the subject of the sentence, it is then followed by sy/sydd. The person is unknown, therefore you need to ask who it is.
If the sentence contains a ‘who’ but is not a question (e.g. the tiger who came to tea), then you don’t use pwy, but you do still use ‘a’, ‘sy’ or ‘sydd’ because the ‘who’ is referring to the subject of the sentence. The ‘person’ (this includes tigers! 
) is known, so no need for ‘pwy’.
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Siaron has explained it perfectly as usual. But anyway, if you have accurately transcribed what is on DuoLingo there, David, then DuoLingo has got it wrong.
Who wants a drink? (with question mark) is Pwy sydd am ddiod? The sydd is there simply because Pwy is the subject, and you can’t put mae after its subject, you have to use sydd instead. Ditto for Who wants to play golf tomorrow which is Pwy sydd yn moyn chwarae golff yfory?
Then Who will be playing golf tomorrow? is Pwy (a) fydd yn chwarae golff yfory?
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Diloch Siaron - thank you for that explanation. It is clear now. I now know - from Garteh’s response that Duolingo was wrong, which was the source for my confusion.
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Diolch Gareth. The transcriptions from Duolingo definitely confused me. Thank you for setting me straight on that issue. When I first opened your book I found it a bit daunting (as a relatively new learner) - but the more I delve into it the more useful I am finding it.
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I take advantage of this thread cause we were discussing (with another Italian learner, ha!) What’s the difference between these examples:
Dyma’r dyn sy’n gweithio yn y siop
Dyma’r dyn (a) allai wneud y gwaith
Why not “sy’n gallu” in the second?
My impression was that in the second example it means he could/might do the work (say, if he’s chosen for the task) while in the first it’s reporting a fact.
But I’m not too sure that’s what actually happening here!
@david-rees-4 do you remember in which Duolingo units these examples were? I wish I could remember where relative clauses and questions are, to have a look at them again but don’t know where they are!
I had those examples from the Duolingo translation site at https://www.duolingo.com/dictionary/cy/translation/en/
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Dyma’r dyn sy’n gweithio yn y siop - Here is the man who is working in the shop (but we can also translate it as “here is the man who works in the shop”)
Dyma’r dyn (a) allai wneud y gwaith - Here is the man who is able to do the work (also can say “here is the man who can do the work”)
Why not “sy’n gallu” in the second? - no reason why it couldn’t be. It’s the same difference as choosing between “Mae e’n gallu” and “Allai fe” - just the short form instead of the ‘bod’ form. 
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Alright!
I guess sometimes we try hard to figure out differences in meaning, when it’s just different form!
Yup, so easy to overthink things sometimes!
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I am so pleased to hear that - and thank you.
The first one means This is the man who works in the shop, and the second one means This is the man who could do the work, for which one could also say Dyma’r dyn (a) fyddai’n gallu gwneud y gwaith; …sy’n gallu… there would mean …who can…
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Isn’t ‘sy’ only used when it’s present tense?