Is the “I” obligatory, essential or optional - I don’t understand where it comes from.
Edit: The penny finally dropped and I guess I was doing a bit of overthinking. If it had been ar ôl i fi, iddo fe etc, then I wouldn’t have queried it and I guess ar ôl i rywun o’r wedi i rywun is just the same sort of thing.
Yes.
Could someone please explain the difference in use between Cymraeg and Cymreig?
Cymraeg is the language. Cymreig is the adjective to describe something that’s Welsh (but not Welsh-speaking / Welsh-language).
e.g. llyfr cymreig - a Welsh book (i.e. about a Welsh topic but not written in Welsh)
llyfr Cymraeg - a book in the Welsh language
Thanks
I know you can also have e.g. Ffrangig, Sbaenig but I don’t see them as much - is it just context (as in, I’m more likely to be reading about Welsh things in Welsh), or is there a tendency just to go with the forms in -eg when we’re not talking about Wales?
And, if so, is that partly to do with seeing the language as co-extensive with the State? Could I (should I) say Iaith Sbaenig ydy’r Gatalaneg?
I guess it is pretty much context-based as to what you see - for instance, in the context of bluebells, you may talk about the difference between clychau gleision Sbaenig and clychau gleision Seisnig.
In my example of books, an original copy of Les Misérables is obviously a llyfr Ffrangeg, but a translation would definitely be a llyfr Ffrangig.
Technically, I think you could say Iaith Sbaenig ydy’r Gatalaneg - although diplomatically (and especially whilst in Catalonia) maybe you should say Iaith mewn ardal o Sbaen ydy’r Gatalaneg 
My old Catalan tutor was quite keen on Catalan being una llengua espanyola - partly, I think, to sow confusion about the term espanyol, forcing people to specify if they actually meant castellà. As a result I tend to say ‘Castilian’ when speaking English, which causes confusion with my partner who speaks excellent (but somewhat South American) Spanish, and tends to see it as meaning ‘posh European Spanish’ rather than just ‘Spanish’…
ETA Maybe Dydy’r Sbaeneg ddim yr unig iaith Sbaenig would work…
Aargh! Right, two quick questions arising from the previous, on points I always seem to get confused about…
1 Am I right in thinking you don’t need yn in identification sentences?
2 Is this a situation where I should be thinking of Weles i mo’r gath (I didn’t see the cat) rather than dim for the negative?
I.e. should I have said Dydy’r Sbaeneg mo’r unig iaith Sbaenig ?
Is there a Welsh idiom akin to “love hate relationship”? (Is idiom even the right word here?)
Something tells me that you only use the “mo” thing with short forms.
The “mo” is from “dim o” so strictly speaking the dim is present.
I saw this and thought, “of course, how silly of me” – and then, because it’s one of those points where SSiW has left me with a feel for it, rather than declarative knowledge, I went and checked in Gareth’s grammar.
So he says that the reason for using mo with a short-form verb is because dim can’t come before a specific object – which means something like Wnes i ddim gweld y gath wouldn’t need mo because you’ve got gweld in between dim and the cat. (even if Weles i mo’r gath might sound better). But with the one I had, there’s nothing between the dim and the yr, so I think the grammar confirms that it should be mo… Paging @garethrking!
No - because the basic formulation is wrong there anyway, since Spanish is not the only Spanish language is an identification sentence, negative in this case but nonetheless identification. So no dydy, and no ddim.
Ahhh… had recognized it as an identification sentence, hadn’t realized they negated differently, have now looked it up. So… Dim yr unig iaith Sbaenig ydy’r Sbaeneg?
No (though I can see that one might think so).
It’'s Dim Sbaeneg ydy’r unig iaith Sbaenig.
In these cases, (and perhaps counterintuitively, but there you are), you take the original affirmative identification sentence, and then negate the whole thing with a preceding Dim (or Nid).
I’m at week 7 and one of the tasks is to create sentences to say out loud, with certain starting points. One of the starting points is "If I can’ BUT what is the word ‘If’?? This is how I get stuck - on tiny things!! 
Most of the time you’d use “os” for if
There is a difference for “if I were able” which is “taswn” but I think for this stage it’ll be “os”
Mwynhau
Diolch - very helpful!
I don’t think the ‘taswn’ (‘pe taswn’ but the ‘pe’, the ‘if’ bit, is usually omitted) includes the ‘able’ bit so you’d say ‘taswn i’n medru’. Open to corrections as always.
No you’re spot on i meant to write “as in” before hand, but missed that bit.
I think it’s because “if I can” was used in the original but who knows 
So for clarity, as @gruntius has said, “taswn i” is “if I were”.