I thought “ac” was supposed to be used before vowels (I know the Welsh alphabet isn’t like the English one, but M is not and never has been a vowel).
For a moment, I thought it might just have been one of those occasional errors thrown in on purpose, but when it was said repeatedly, I knew I had to ask: why “ac” and not “a” before “mae’n”? 
Myfanwy the Picky
I was going to say because it is easier, but it turns out that mae is one of the words that is an exception to the rule.
Waw, diolch! 
Thanks for taking the time to answer.
I think it’s because in literary (and older) Welsh, the form of the verb is ‘y mae’, rather than just ‘mae’. You’d need an ‘ac’ before ‘y’, of course. Even though the ‘y’ has disappeared in the modern/colloquial language, the ‘memory’ of it lingers, hence ‘ac (y) mae’, rather than ‘a mae’.
I don’t know the answer to why. It may be as explained above, with “y mae” but then a + y also becomes “a’r”. But then I thought “a ha, Anthony, the y is a different y”…and then my head started to hurt.
But irregardless of all that, it’s an exception to the norm in modern Welsh, not a deliberate mistake.
PeterG got it bang on - that’s exactly the reason 
It’s as @PeterG says. Once upon a time it was ‘ac y mae’. The ‘y’ has disppeared, the mutation, treiglad, remains. It happens quite often as the language changes. But, as Iestyn and Aran say, ‘Don’t worry about it’.
You’ll find that there are a few other words that work like ac mae: ac wedyn, ac sydd, ac felly, ac mor…
This is mainly written Welsh, for which a good reference is “Ymarfer Ysgrifennu Cymraeg” by Gwyn Thomas.
May I give you 27 “likes” for that informative answer? 
x 27
And another book for my library!
When I was doing Middle Welsh, given that scribes/authors often varied between spelling things i or ei or y, we ended up with about 4 or 5 different i’s and maybe as many as 7 different y’s, all of which could only be told apart by their interaction with other words around them (and, y’know, context & guesswork). And then you come to the poets, who seem to just miss all of those little words out, and the swearing and the guesswork actually increase tenfold…
ETA: So, yeah, it’s a different y. I think it might be y(d) rather than y®.
Can I toss in a question about ‘h’? I thought ‘ac’ was used in front of it because so many folk lose it, but I recently came across an example of ‘a h…’. Which is right?
It’s normally just “a” before h. Like “…a hefyd” “hogyn a hogan”.
Ah, interesting. I knew it wasn’t “y = the”. So is it closer the the “y” as in “that” “y ddylwn i”?
Hm. The short answer is, “…maybe?”
The long answer is, “I don’t know, and I’m currently trapped at a children’s party, but when I’ve found an hour or so to confuse myself with both a Middle Welsh and a Literary Welsh grammar, I think you’ll find the answer is… Maybe?”
(I’m actually pretty sure that it’s the yd- of verb forms like yd wyf i -> dw i and maybe also of ydy instead of Southern yw, but beyond that I’m just basically making stuff up as I go along!)
Haha, I’m happy with “maybe”. After a while “why” questions don’t help me speak the language. I doubt I’ll ever become a writer in English, let alone Welsh.
Thank you for your answer. Enjoy the party
Just to expand on my sleepy 01:00 answer. This is what I found in Gareth Kings Modern Welsh Dictionary - the exceptions that we are likely to come across will be:
“Parts” of bod, such as mae, roedd, bydd, etc; also: fel, felly, mewn etc.
I’m no expert, and haven’t studied historical forms of Welsh beyond a cursory glance, but I think it’s neither y as in the definite article (‘the’) nor y as in the conjunction (‘that’). Rather, it’s a ‘meaningless’ affirmative marker used before the present and imperfect forms of the verb ‘to be’ in high literary/hen ffasiwn Welsh.
So present tense (I am, you are, etc), with derived forms…
yr ydwyf, yr wyf > rwyf > rydw i > dw i > wi, etc
yr ydwyt, yr wyt > ®wyt ti
y mae > mae e/o, hi
yr ydym > rydym > rydyn ni > dyn ni > yn ni, etc
yr ydych > rydych chi > dych chi > ych chi, etc
y maent > maent > maen nhw
And in the imperfect (‘I was, you were’, etc)
yr oeddwn > roeddwn i > ro’n i > o’n i
yr oeddit > roeddet ti > ro’t ti > o’t ti
yr oedd > roedd e/o, hi > oedd e/o, hi
yr oeddem > roeddem > roedden ni > ro’n ni > o’n ni
yr oeddech > roeddech chi > ro’ch chi > o’ch chi
yr oeddent > roeddent > roedden nhw > ro’n nhw > o’n nhw
So you can see where the ‘r’ comes from in the imperfect forms (roedd, etc), though this is usually dropped in speech and very colloquial writing (something that a lot of self-tuition/grammar books don’t seem to acknowledge).
I don’t think this affirmative marker is/was used with other tenses but I stand to be corrected on that.
(I know this kind of philological musing isn’t really the SSiW way but I find it interesting, even if no-one else does!)
Heh! I know it isn’t and I’d probably be better off not asking such piddling little questions, but as an inkslinger interested, like you, in philology, I can’t help myself.
For all of you who have answered my original question and given me interesting things to think about… diolch yn fawr iawn iawn iawn!!
Croeso, sororp. Glad I’m not the only one to find such stuff interesting!
Now, if only I could actually speak Welsh…!
Right, I’ve had time to confuse myself thoroughly now, so here goes!
There seem to be at least three different but similar pre-verbal particles in play here.
Y(dd), usually spelt yd in Middle Welsh; y before consonants, ydd before vowels. It’s a more-or-less meaningless affirmative particle that often appears before verbs – pretty much like Modern mi or fe. See Middle Welsh Yd aeth, think Modern Mi aeth o. My Middle Welsh Grammar says it “is found before the verb at the beginning of a sentence,” but also that it “occurs before a verb to introduce a […] relative clause.” It’s also the fourth word of the Mabinogi: Pwyll Pendefig Dyfed ydd oedd yn arglwydd ar saith cantref Dyfed…
Y®/ry, which did more or less the same jobs as ydd (affirmative particle, also used in relative clauses) and “spread at the expense of ydd” in later Middle Welsh. This is the r- of rydym etc. that @PeterG was referring to. It is also the one that my Modern Welsh grammar refers to when it says that y (or “yr before a vowel or h”) is used in relative clauses like Pwy yw’r ferch y torrwyd ei breich? (“Who is the girl whose arm was broken?”). I think that this is the same particle that @AnthonyCusack was referring to, but I’m not altogether sure
Yd, usually spelt yt in early Middle Welsh, used before consonants (plus soft mutation) in older Middle Welsh in much the same way as ydd, confusingly enough.
Given that the grammar says that yd/yt was used before consonants, it’s not entirely clear to me whether or not it’s the same yd/yt that is sometimes found in front of Middle Welsh forms of the verb bod, and is now an absolutely integral part of the verb in the Modern language. In other words, Middle Welsh ‘I am’, ‘we are’, ‘they are’ are normally wyf, ym, ynt; spoken Welsh has forms based on yd+wyf i (dw i), yd+yn(t) (ydyn nhw); more formal Welsh has forms based on ry+yd+wyf (rydw), ry+yd+ym (rydym), ry+yd+ynt (rydynt), although my grammar of written Welsh seems to think that forms without yr are actually even more formal, e.g. ydym or even ŷm! (Yikes!)
Oh, and there is (or might be) another one.
There’s a form of the verb ‘to be’ in mediaeval and ever-so-formal Literary Welsh that corresponds to the formal impersonal cenir (‘someone sings’, ‘singing happens’) of other verbs: ys. (It’s actually the same word as Spanish es and English is.) If you join this up with the old ef (= Modern fe/fo) you get sef (short for ys ef), which still seems to have actual currency as a way of saying ‘that is’, ‘i.e.’ It also got joined onto an old word for ‘if’, o(t), to give modern os; and it gets joined onto some sort of relative pronoun ydd (which may or may not be yet another different but similar word) to give us sydd for ‘which is’.
I think that’s about all for now…