The rules for mutations in the formal language are well established and mostly adhered to. In the spoken language there is some variation in how consistently the rules are followed. To quote Aran “Welsh speakers are wriggly things who don't necessarily stick to the 'rules”.
For learners this is disconcerting at first, but in time it just becomes part of the landscape and one's style evolves.
The purpose of this article is not to give the formal rules for mutations which are well explained in books on grammar, but to discuss deviations, real or apparent, from the rules.
The idea for the article came from a thread in the SSiW forum discussing forms of gwneud. Some of the other queries have turned up in the forum as well.
Gwneud. The short form past tense of gwneud is gwnes i, gwnest ti, gwnaeth o, etc
If it is preceded by the particles Mi or Fe as is common in speech, it is mutated: mi wnes i, mi wnest ti, mi wnaeth o etc. and this, although grammatical, is less formal. But quite often in speech the particle is omitted whereas the mutated form is still used, notably when gwneud forms the past tense of other verbs: wnes i fynd, wnest ti fynd, wnaeth o fynd. As the w may be scarcely audible it is commonly omitted in informal writing: nes i fynd, nest ti fynd, naeth o fynd etc.
Non-mutating words. A number of words are immune to mutation for a variety of reasons:
Moyn (SW) “want” - because it is a contraction of ymofyn where the y shields the m from mutation: Dw i'n dal i moyn fy nghinio
Gêm, golff, gôl, gard , gitâr, gât – short loan words not fully absorbed into the language, where loss of the g would change the character of the word too much. Note that while these are immune to soft mutation, they may undergo nasal mutation. Dw i'n canu fy ngitar bob dydd.
Words already mutated therefore not mutating again:
Beth? – what? Am beth? – for what?
Ble? – where? I ble? – where to?
Note that double mutations are occasionally encountered in words that are habitually used in mutated form to the extent that the original mutation is forgotten:
pobl becomes y bobl so often that one can hear things like o fobl Cymru
Words only mutating sometimes:
Byth – “never, ever” Cymru am byth
Fyth – “even” Mae'r tywydd yn oerach fyth heddiw – the weather is even colder today
Adverbial phrases of time are not always mutated in speech
Dw i'n yfed gwin bob dydd may be heard as Dw i'n yfed gwin pob dydd
Words shielded by a preceding y in the formal language which is not normally spoken
eg (y) mae
People's names are not usually mutated
Weles i mo Gareth neu Dafydd
Sentences like Beth ydy o'n wneud? are often heard. THe apparently random mutation of gwneud is due to an ei in the sentence which the formal language requires but which is often omitted in speech: Beth ydy o'n ei wneud?
Nouns used as possessives (ie genitive case) after a feminine noun do not mutate, whereas mouns used adjectivally do mutate: Llyfrgell Genedlaethol Cymru, Pobl Cymru. There is a good discussion of this point on the forum