absolutely, you’re definitely a ‘ti’ with me! 
yes, well spotted, “maen nhw” is correct - I missed that one!
It should be “Maen nhw’n dod o…” although, to be fair, because in speech “maen nhw”, “mae’n nhw” and “mae nhw” all sound exactly the same, the latter two are only incorrect when written down 
In this context, you’ll often hear ‘athro oedd o/hi’ - it feels natural to place the emphasis on the ‘teacher’ bit… 
On a similar note - “I will go”: Wi am fynd.
My 1990s Dosbarth Nos work book has “(D)wi am” as one of the four options for this future tense use. Yes, I know, I’m a slow reader
Gareth King’s dictionary has it as want to etc.
GPC as want to but also as will (about to/am going to) type meaning, but it’s latest example is from the 1930’s.
Is it common in speech for will, please?
I can’t say I’ve noticed it being used for ‘will’ in speech (but maybe that’s just me not noticing), but for ‘want’, yes, that’s very common. Maybe someone else will have noticed it for ‘will’
Thanks, Siaron.
I can’t say I’ve noticed it, but I probably will now.
The other forms given were straight forward:
Wi’n mynd I
Bydda i’n
&
Short form eg ysgrifenna I
This is really about meaning blur in English - if you really think about it, ‘I want to go’ and ‘I will go’ aren’t really all that different from each other - you’re used to the fine detail there, as an English speaker, but the underlying concept has a fair bit of overlap. So in Welsh, you can say that ‘dwi am’ means ‘I want’ - except of course it doesn’t, really, because ‘I want’ is ‘dwi isio/moyn’ - so ‘dwi am’ is another way of expressing a desire - and desire/intention is overlap territory. So yes, you will hear ‘dwi am’ used when the person isn’t particularly trying to emphasise desire…
That’s great thanks. Aran. I think I get it now. I’ll be looking out for the context. I take your point about the overlaps within English too.
Thanks for the tip Aran!
Another question on the conjugations of future Mynd that I linked to above - we have ‘a i’ (i will go) and 'ei di (you will go) etc. Are these forms strictly for ‘mynd’ or do they appear with other verbs too?
Hi Nadip
Someone else will be along, probably during the week.
In the mean time, here’s my novice take on it.
Yes the ones that you showed are for Mynd (go).
The other three irregular verbs, dod (come) gwneud (do/make) and cael (get) are similar to Mynd, but you’ll need to hear them.
All of the other verbs are more or less standard in that if you want to use the short term future endings, they generally share the same set as each other. Again this is all in the challenges.
Don’t worry. I’ve made it sound worse than it is.
Sorry, I forgot to say: Bod (to be) is in a class of it’s own so has special endings.
As John said, those are specifically for ‘mynd’ - short forms are used for other verbs - more so, Iestyn assures me, in the south than in the north, where it’ll usually be for mynd/dod/dweud/gwneud/cael…
Thanks. I can’t seem to find a list of long forms. Would they be something like
Mi fydda mynd I…I will go
?
Almost -
Mi fydda i’n mynd (i) = I will go (to) / I will (be) going (to)