So, I FINALLY have time to start “studying” again! 
To that end I’ve begun the “new” course (which I love!) One great thing is it’s exposing all these little holes in my knowledge I wasn’t aware I had!! For example, “a” and “ac”. I thought I understood when to use “ac”, but I’m hearing examples that don’t follow what I thought I knew! So, what’s the rule? When exactly does “a” become “ac”?
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…when the following word begins with a vowel or an H.
Also, when the following word is “mae”, but I’m not entirely sure if the same applies to all words beginning with M!
Croeso nôl, gan y ffordd! 
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diolch! I had the “before vowel or h” part–but I somewhere along the line I missed the “before mae” part!
As faithless says, it’s vowels and consonants.
But “ac” before “mae”.
There used to be an “y” before mae, which sort of explains the “ac”, but there used to be an “y” before a lot of things, so really it’s best to just think of “ac mae” as an exception.
Probably no more help than what faithless said, but hope it helps a little…
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ah! actually that does help! I had been thinking of “a” and “ac” the way “a” and “an” work in English, so having 'ac" before “mae” didn’t feel logical to me (which is probably how I’ve missed it all this time!!) However, if there used to be an “y” before “mae” that make more sense to me! Thank you!!
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Ahhhh… That would explain it then!
I always wondered why “ac” was used before “mae” and not “a”. That makes a lot more sense!
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I think it’s “ac” before “mor” also. I’ve just read a sentence in a book that includes “mor hen ac mor ifanc”. Blame M. S. Ros if it’s incorrect. 
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No, it’s only before ‘mae/maen’, and before affirmative particle ‘mi’; the past tense ‘roedd’ also usually takes ‘ac’.
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