Is the stress in “ac ati/and so on” on the ‘a’ in ati—a bit like an ‘â’—or a short stressed ‘a’ or on the ‘i’?
… ac Ati… 
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It’s on the a in ati.
Whilst there are exceptions (aren’t there always!), in most Welsh words of more than one syllable, the stress is on the penultimate syllable.
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Thanks Aran.
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Thanks Siaron. I just thought that this just might be one of the exceptions.
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If I remember correctly (and no guarantee of that!) the exceptions tend to be with either some words ‘imported’ from another language (e.g. ambiwlans) or words with an accented letter at the end (e.g. cadarnhâd) - or both (e.g. jiwbilî - jubilee).
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Yes, of course
I stand by my ‘no guarantee’ 
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And, most ironically, the word Cymraeg!
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… or ending in “og”
I’m not sure about that one John to be honest, I can think of a few that don’t anyway like ‘miniog’, ‘gwyntog’, ‘caregog’, etc. Although I am wrong a lot.
‘Ymlaen’ is another I can think of though.
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Actually, the more I think of it, doesn’t Siaron’s penultimate rule mostly apply to words of three syllables or more, that aren’t compound nouns? The two syllable ones seem to have minds of their own to my thinking. For example, gwybod and deallt seem to follow different rules. I’m trying to think of a clever pun to include knowing and understanding, but I’m not having much joy at the minute.
It seems with the last couple of posts that I’m just disagreeing with you for the sake of it @JohnYoung but believe me, I’m not.
The ‘stress on the penultimate syllable’ rule definitely includes two syllables and, to my ear, include the two words you mention. There are exceptions of course, as mentioned above, like compound nouns, imported words, -hau words, Cymraeg, etc. (Ymlaen is a compound from ‘yn blaen’).
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That’s great thanks, @gruntius. TBH I was just “pronouncing” words in my mind for 5 minutes at the end of lunch break when I posted the nonsense, above. You are winning me around on “deallt” now
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Although with ‘deall’, as you may or may not know, in the north we say ‘dallt’ so only has one syllable anyway. 
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I noticed that pretty much all of the exceptions mentioned by @gruntius and @robbruce (not sure about @siaronjames’ cadarnhâd, mind you) end in diphthongs, and I was speculating a bit about whether they were somehow being scanned as if, say, the a and the e of Cymraeg counted as separate syllables – much like how llyfr counts ‘officially’ as one syllable (even though it isn’t), and so dy lyfr di is your liver and not your lover.
And then I gave up speculating, and dug out the Middle Welsh grammar again, and had a look
…and found three different situations. The grammar says that in words like Cymraeg, a sound had been lost between the a and the e, and so in poetry they counted as two separate vowels even though they were already pronounced together as one in the Middle Welsh period. In other words, cynghanedd was complicated enough and conservative enough that Cymraeg and Frangeg would count as having the same vowels for purposes of rhyme. In fact, the GPC lists Cymräeg alongside the normal form, surprisingly. But anyway: if you count the a as if it were a separate syllable, then the stress is actually going exactly where you’d expect.
(I was wondering how late this change from a+e to ae actually happened, given that you also get mediaeval spelling like cahel for cael, and given that it seems easier to explain modern pronunciations like Cymrâg or @Nicky’s llaaath if they were just dropping the e off a+e, but the grammar says it was already a diphthong and I certainly don’t know enough to quibble with it…)
Secondly, there’s words like casáu and parhau. The Middle Welsh grammar says that these are formed from the root of a noun or adjective (Mae’n gas gen i) with -ha added; sometimes this leads to other changes (e.g. bwyd + -ha = bwyta ‘food’ -> ‘eat’); and sometimes the verb-noun is made by adding a -u (mwyn ‘nice’ + -ha-u = mwynhau ‘enjoy’). I’m guessing, from the way that the grammar actually spells out digrif-ha-u (‘pleasant’ -> ‘please’), that these might get treated in poetry as if they had the same vowels as, say, cymharu. At any rate, it turns out that there’s at least one more familiar one – if the stress shifts away from the ending onto the syllable before, the -hau gets weakened (like an + hawdd turning into anodd), and mediaeval blinhau (from blin, ‘tired’) turns into plain old blino.
And then finally there’s ymlaen which (a) has another of these diphthongs and (b) as @gruntius pointed out, is a compound of yn + blaen. But I wonder if it’s not also significant that the first bit of the compound is just the preposition yn… In English, a word like ‘redhead’ (person with red hair) or ‘blackhead’ (spot) or ‘warhead’ would all get the stress on the first element of the compound; but a word like ‘ahead’ (from mediaeval ‘on’ + ‘head’) naturally gets the stress on the second element. Can anyone think of examples of other compounds in Cymraeg that’d show whether or not this is relevant?
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There’s all the verbs with ‘ym’, e.g. ymweld, where the stress is also expectedly unusual, similar to your ymlaen example, but from the reflexive prefix ym-, not yn, and many other prefixes such as dad/dat- as in datblygu from dat+plygu where stress is also on the usual syllable in the root of the compound. So I’d say that compounds created with prefixes tend to follow this pattern.
With regard to -hau, as in parhau, there is the adjectival suffix -ol which makes parhaol, where the stress is on the usual syllable (a), in other words, the au in -hau is not originally a diphthong by the looks of it
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Interesting point re. dipthongs @louis. It seems (since a syllable is dependent on sound created rather than the number of vowels) that there could be some debate as to whether a dipthong comprises a pure single sound or two sounds run together at speed and so forming one or two syllables. 
. Maybe someone knows the answer.
Just checked and it seems that the rule is that a dipthong counts as one sound. I think that they often sound like two sounds at speed though.
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My understanding is that a diphthong is a vowel sound that is produced initially with one set of vocalizations (front, open, high for instance) but changes one or more of these characteristics during production, and is therefore often written as a combination of two vowels, e.g. au, oe, ae in Welsh, although a real diphthong is considered one sound.
In Richard’s example of au in parhau, I think that originally this was not a diphthong, but a sequence of two separate vowels, hence the stress being not on the ‘par’, but on the ‘hau ‘
Only of historical relevance,of course.
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