Ok…the following is confusing.
Huw yw athro.
Huw ydy athro.
Mae Huw yn athro.
What’s the difference? Apart from emphasis, when ‘Huw’ is the start of the sentence (I’m guessing) what is the difference between these three examples? When do we use ‘ydy’ and when do we use ‘yw’? Thanks.
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Yw and ydy are pretty much entirely interchangeable. You’d use yw/ydy as the verb if the subject (Huw in the example) is first, and you’d use mae as the verb if the subject is second.
From my understanding there is no real difference, just a regional thing. If I remember correctly, in the north you’ll hear “ydy” predominantly, while “yw” is used in the south. (Only in sentences, though. As a form of “yes”, as in “yes, he is” you’ll also hear “Ydy” in the south.)
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So helpful. Many thanks.
Would not the third sentence, ‘Mae Huw yn athro’ be like ‘Hugh is teaching’, which is a slightly different meaning.
My understanding is also that ydy and yw are the same, with ydy being more ‘Northern’ and yw more ‘Southern’
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Well,
Huw yw/ydy athro is wrong, and indeed not Welsh - if you want to say Huw’s a teacher, it’s either Athro yw/ydy Huw or Mae Huw’n athro, (and those two are identical in meaning, @Y_Ddraig_Las - in answer to your supplementary query)
If you want to say, however (using an identification sentence):
Huw’s the teacher
that would be
Huw yw/ydy 'r athro
(Huw is a teacher is not an identification sentence, you see - nothing is singled out).
And yes - yw is broadly south, and ydy broadly north. But with questions it is Ydy…? everywhere (EXCEPT that some southern regions even use Yw…? here as well)
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Oh, and also don’t forget one last pattern:
Athro yw/ydy Huw
This means Huw is a teacher (i.e. and not a bricklayer)
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Many thanks Gareth for, once again, cutting through to the clarity of the thing. 
Just a final question. You say Huw yw athro is wrong - but would it be OK to say, Athro yw Huw, Thanks again.
emphasises that he’s a teacher not a physicist (
)Fab - many thanks everyone.
Hello, I know I am coming to this four years later… maybe five… but in the sentence Mae Huw’n athro which Gareth explained meant “Huw is a teacher”, what does the “yn” mean? (I am assuming that the " 'n " is an “yn”
Thank you! and also, thank you for this whole thread, which is super helpful
It’s just part of the construction, so in that sense it doesn’t mean anything by itself - when you have a sentence beginning with a form of bod (mae), followed by the subject (Huw), followed by either a verbnoun or an adjective (in this case athro is the adjective describing Huw), you need to link it with an yn.
thank you! (And also for your lightning speed) - I thought “author” was a noun here, so now I realise why I wasn’t understanding - thank you!
Just to avoid confusion (in case autocorrect has jumped in), athro = teacher (awdur = author) 
While athro (and indeed awdur!) are nouns, in this sentence construction they are being used as adjectives, which is why the yn pops in there. If you said the same thing in a focused way - Athro ydy/yw Huw, it’s being used as a noun, so no need for yn in there.
This is brilliant, thank you. I am so happy I am learning Welsh!
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A ninnau! So are we! 
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And what about: is she swimming? - ydy hi’n nofio (ydy)? But: how often is she swimming? - pa mor aml mae hi’n nofio (mae, not ydy)?
Why?
Because in Is she swimming? the question is whether or not she is swimming (we do not know which till we get the answer), whereas in How often is she swimming? the question is not whether or not she is swimming (we know that she is - therefore mae), but the secondary and separate issue of how often.
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