Thank you so much for your detailed reply, Siaron, and assuaging of my anguish! I didn’t mean to imply that I thought it was a relative clause because of the ‘implied that/which.’ My apologies. My more complete initial thinking step was: there is an implied that, but that that can just as well be a which, therefore there is an implied that/which. It seems my fundamental problem is believing, without exception, the statement on page 43 of GK’s Intermediate Grammar: “If you are dealing with a relative ‘that’, then you will find that you can replace it by ‘which’ and still have the sentence make sense.” Thus, I excluded the sentence being a subordinate clause with ‘y’ as the appropriate answer (Unit 12 of Intermediate Grammar) simply because this sentence under the microscope made sense using which. I’m not very good at putting what I am trying to learn of grammar into conversational practice, but I don’t fret about that, and I do rather enjoy the challenge of asking myself ‘Y?’ over my early morning pot of tea!
I have to say, this is one element of Welsh grammar I have never got my head around, so I take my hat off to you for grappling with it (and to Siaron for having mastered it!)
I would now consider myself a fluent speaker, and I do my job through the medium of Welsh. I always get one of our Comms team to edit/correct anything I write that is going to be shared publicly, and they don’t seem to be picking me up on this on a regular basis, so I must have imbibed the rules somehow. But if you were to ask me, I don’t think I would be able to give a coherent answer as to why, beyond “that’s what feels right”!
The only thing I would add is that the English “rule” as to when we use that or which (defining / non-defining clauses) isn’t all that helpful. This is one I do know well, so I tried to map it onto y/a but I couldn’t make that work (and most people don’t keep to the distinction in any case).
I was cruising along quite happily just using the “it feels right” system, until I went to an Uwch class and was required to do exercises putting the correct word and explaining why. I got super confused and went to pieces for a while, totally unsure of what I should say. I only really got around it by going back to Plan A and just reading as much Welsh as I could, saying any sentences with ‘a’ or ‘y’ joining phrases out loud, until I relaxed again and stopped worrying about it.
Now I only have to panic if someone learning asks me about it! 
I promise I won’t ask you on Slack now @Deborah-SSi - it was on my list for when I next joined your Q&A session!! Thank you everyone for your responses, and I hope I haven’t confused or daunted too many other learners. I think the ‘just keep reading’ and ‘don’t worry’ approach is exactly right, especially because I’m learning Welsh for interest and fun and shall never have to worry about writing ‘correct grammar’ (the idea of doing formal classes is far too scary for me - and I have distance and time zones as convenient excuses!). Even though it does ‘do my head in’ rather more frequently than I would like, I also do enjoy nibbling away at Gareth King’s grammar exercises over my morning pot of tea. Also, that activity is a form of reading with the extra challenge (benefit?) for me being that his grammar books are somewhat weighted towards De while I’ve been learning Gogledd. Diolch yn fawr, pawb!
Not so much a rule, as a suggestion by Fowler et al over a century ago: wouldn’t English be both more precise and more elegant if we were to consistently… etc.
Funny story: back in 2017, I was using Duolingo to revise my Greek, and I got into a bit of a discussion with one of the volunteer maintainers over a sentence in English which I had had marked wrong, because I had failed to observe this supposed distinction. The maintainer was a late-middle-aged American man who, like many well-educated Americans, had been taught such stylistic preferences as if they were grammatical rules, and wasn’t at all convinced when I argued that it was merely something made up a century ago. Fed up with this kind of nitpicking, I wondered how well Duolingo would work for me if I were trying to learn a language from scratch, rather than revise one I reckoned to know already. I looked at the list of what they offered at the time, and thought, “Well, I’ve always meant to learn Welsh…”
Yes - “zombie rules” we call them in the editing community (because they will never die…) But that one was particularly deeply embedded, and many clients insisted on it.
But I love the fact that’s what brought you to Welsh! Almost makes it worth it…
That pesky ‘taswn’ when used with eisiau—in reviewing Level 2, Lesson 24, around the 6:50 min mark I still cannot figure out the latter part of the sentence ‘I would tell you if you needed’. Is it ‘…tasai eisiau arnot ti’? And how does this construction work please? Do you just substitute tasai for mae, and arnot ti for i ti?
Thank you.
I’m a learner so I hope this explanation helps!
I think there is a difference here between the challenges and the automagic SSIW app.
In the latter I would say “bydden i’n dweud wrthot ti (ta)set ti angen” (in automagic version they tend to use angen for need).
But when using “eisiau” for need ( nothing wrong with that) you could say “bydden i’n dweud wrthot ti 'set ti eisiau”
That makes sense. Thank you.
There’s a subtle difference between tasai eisiau arnat ti and 'set ti eisiau (both pronounced isie in the south)
In the first you’re saying literally “if there is a need on you”, i.e. “if you have a need” - remember that when you use eisiau for “need” in the south, it has 2 constructions - mae eisiau i fi xxxxx “I need to do xxxxx” or mae eisiau xxxxx arna i “I need a xxxxx”
e.g. mae eisiau i fi fynd nawr “I need to go now” or mae eisiau car newydd arna i “I need a new car”
If you say 'set ti eisiau you’re not using the southern ‘need’ construction, but rather the more general meaning of “want” - so that’s closer to “if you want”, than “if you need”.
Yes, I did say it was subtle 