I’ve had trouble with this subject for ever but I hope there’s an easy answer!! When does one use ‘dim’ and when ‘nid’? For example, is it ‘dim fi’ or ‘nid fi’? Is ‘dim’ used for ‘no’ and ‘nid’ for ‘not’ or is it not that simple?
I KNOW there’s someone out there with the answer!!
Dolch yn fawr.
It’s “nid fi” but i don’t know why, sorry!
The meaning of “dim” depends so much on context I’m not sure I could explain it.
It’s a negative emphasis. So it’s kinda similar to “no” but can’t be translated as the English “not” when used with a verb.
“Nid” is not. “Nid wy’n gofyn bywyd moethus” - I am not asking for a luxurious life
Dw i ddim yn gofyn am fywyd moethus - I am not asking for a luxurious life.
See…told you I couldn’t explain why!! Sorry! But I’d always say “Nid fi”
Best I can think of is that “nid” is a verb that covers “to not be”, while “dim” is a negation. It’s a question of emphasis for the most part (except in music where it’s probably more a question of rhythm) - “nid fi sydd ar fai” emphasises the speaker, while “dw i ddim ar fai” emphasises the negation. The first one implies that someone is to blame, just not the speaker, while the second might be used when nobody is to blame.
At least, that’s my take on it; it could be entirely wrong…
That was my gut feeling but you’d write “Nid ydw i” wouldn’t you? Which would have “ydw i” as the form of bod? And I think “wy’n” from calon lan is “bod”…
@garethrking could you help us please?
No, because the way I’m reckoning it, “nid” is essentially the opposite of “bod” - you use one or the other, not both. It can be a little confusing, since English doesn’t have a verb for “to not be”, but that’s basically the way it seems to work so far as I can tell. So the same way you would use “dw i” for “I am”, you could use “nid fi” for “I am not”.
Not quite sure where wy comes in, if I’m honest - it could be similar to how some people in the south use “fi” in place of “dw i”.
Or, of course, I could be barking up entirely the wrong tree.
When used in the emphatic pattern e.g.
Nid fi sy’n mynd / dim fi sy’n m_emphasized text_ynd
You will here both being used, dialect and formality etc which one (with the extra varietyof ‘nage’ being used in this construction in the south). “nid” is technically more correct and so would usually be written.
Nid wyf vs dw I ddim is the formal written form compared to the spoken form.
‘nid’ is not a verb in itself but a negative marker
No, Hector - as Dai points out, nid isn’t a verb of any kind, it’s simply the negative particle, like English ‘not’. It’s ni usually, but nid before vowels (broadly), and also in front of anything that’s not a verb.
And it’s very literary - practically always replaced by dim in speech and in normal styles and registers.
Ni fyddaf = Fydda i ddim = I won’t be
Nid ydwyf = Dw i ddim = I am not
Nid fi ddwedodd hynny = Dim fi ddwedodd hynny = It wasn’t me who said that
Well I found a use for it recently (i.e. noticed it either on Radio Cymru or S4C).
“Dim ond” means “only” doesn’t it? So how do you say “not only”?
“dim dim ond” sounds clumsy, and probably wrong.
Then I heard “Nid dim ond…” and a light dawned 
Nid ydw i - is the “correct” form of nid fi, so it doesn’t replace “bod”
In Middle Welsh dim means ‘anything’, but it also gets used frequently with negatives to reinforce them, and so it winds up coming to mean ‘nothing’ or ‘not’: nid wyf i -> nid wyf i ddim -> dw i ddim. Also (I’ve just been looking at my Middle Welsh grammar!) dim onyt started off being used only in negatives - they give the example Nit oed dim onyt croen ac ascwrn (Nid oedd dim ond croen ac asgwrn - He was nothing but skin and bone) - which is how ‘nought but’ winds up meaning ‘only’. Also also, mo apparently comes from dim o, so Welais i mo’r gath = ‘I saw nothing of the cat.’
(Also also also: the evolution of a word from meaning ‘anything’ to just ‘not’ is a thing that languages do often enough for it to have a name - it’s a Jespersen’s Cycle, and Welsh, along with French, is a classic example.)
also in “pob dim” - “everything”
In the Middle Ages, Dutch had “en…niet”, e.g. “ick en seyde het niet” (I did not say it), these days, the “en” has gone, so also a Jespersen Cycle there. Afrikaans still has the double negation “nie…nie”
:long-winded reply only tangentially about Welsh: 
I think that the defining characteristic that makes something a Jespersen’s Cycle is that a word starts off as a positive but comes to acquire a negative meaning. [EDIT - No, I’m wrong, and you’re right. But the comparison between French & Welsh on the one hand, and Dutch & English on the other still stands.] It strikes me that the Dutch change probably doesn’t count, because I take it that the niet was always negative. In fact, it looks more like English to me: Old English had a word wiht that meant ‘a thing’ (used by Tolkien in The Lord of the Rings for the Barrow-Wights), from which we get āwiht, āht ‘anything’ (MnE ‘aught’) and nānuht, nāht ‘nothing’ (MnE ‘naught’ and ‘not’ - from ‘none’ + ‘thing’). Hence:
Ic ne sǣde - I did not say
Ic ne sǣde nāht - I didn’t say nothing
I seide naught (Middle English)
I said not (Early Modern)
I did not say
I don’t know Middle or Old High German, but nicht looks so much like OE naht that I assume it’s gone through the same process.
But what’s happened with Welsh is more similar to French: in Old French a negative such as jo ne dis ‘I did not say’ can potentially be reinforced by a whole bunch of other words (which you can choose does depend a bit on the verb) - ‘a step’ (pas), ‘a crumb’ (mie), ‘a person’ (personne), ‘a thing’ (rien). Some of these are still kind of in-between in Modern French - personne is ‘no-one’, but une personne is still a person - but rien, like Welsh dim, has pretty much completed the shift from ‘something’ to ‘nothing’.
Interestingly,in this Spanish follows the same pattern as Dutch and English (and so gets trotted out to counter pedants who claim ‘I didn’t say nothing’ must mean that you did say something), whereas Catalan patterns with French:
No he dicho nada - ‘I didn’t say nothing’ (Spanish)
No he dit res - ‘I didn’t say anything’ (Catalan)
But to bring it back to Welsh – he says, aware that we’re veering madly off-topic – if you look up res in a Catalan dictionary, it has two definitions: 1. (in questions) something/anything; 2. (in negatives) nothing. And this makes sense, in that the difference between ‘Have you got anything?’ and ‘Haven’t you got anything?’ is effectively just one of emphasis, whereas the difference between ‘I’ve got something’ and ‘I haven’t got anything’ is one of fact. In a question, asking ‘Is X true?’ and asking ‘Is X true (or not)?’ and asking ‘Isn’t X true?’ are all effectively more or less the same. So res can mean ‘anything’ in a question, but ‘nothing’ in an answer, in the same way that asking Wnei di nôl panad i mi, na wnei? can mean the same as ‘Will you get me a cuppa?’
Wow!! - and there’s me hoping that there might have been a simple answer!! Thanks to everyone who’s responded to my original question and replies - I think it’s going to take me some time to take all that info in but, at least I have a starting point now.
Diolch unwaith eto.
I suppose, for completeness, it’s worth noting that the d that is found at the start of such phrases as dydy e ddim or do’n i ddim is a vestigial remainder of the original nid.
Thank goodness for that! I’ve been looking for an explanation of this for about an hour. Now I can relax and come back to it when I’m feeling ready to be literary!