1)Personal names do not mutate in the spoken language and hardly at all in the modern written language. Forms such as i Fair, i Ddafydd or i Lywelyn suggest quite a formal context. The first translation of the bible in Welsh has plenty of examples as does much Middle Welsh although even here it is not consistent. Unless you're writing quite formal prose (or poetry) it is best not to mutate names. Having said that, my impression is that names of historical characters are more likely to be mutated in writing , eg gan Uto'r Glyn, gan Ddafydd ap Gwilym, am Ddewi Sant, am Werful Mechain, but I don't believe this is mandatory. Incidently, Welsh placenames are consistently mutated in the written language but foreign placenames are generally not unless there is a well known Welsh form eg Manceinion, Llundain. Occasionally forms like o Baris, i Ferlin, am Fosco are found but not with lesser known places. Writers like Ambrose Bebb often gave foreign placenames a Welsh form and then felt entitled to mutate, eg i Brofans. In the spoken language the expected nasal mutation after yn is sometimes now omitted or even replaced by the soft mutation. This sounds horrific to purists naturally, eg yn Gaerdydd. 2) The word fy does not change. However in the spoken language the word is often substituted by the form 'y(n): 'y nhad 'y nghath i 'y mhres i yn afal i etc You will sometimes find these commonly used forms in modern novels. 3) In fact eu and au are not identical in pronunciation. There is some regional variation but generally I think you can say that au retains the a sound in the first element but the e of eu is nearer the shwa sound. Check with a native speaker! 4) Yes the stress will move. In a word like mynydd the stress is on the myn- and in mynyddoedd it is on the -ydd-. Same goes for words like ffenest/ffenestri; gorsaf/gorsafoedd; gwleidydd/gwleidyddion etc etc. With words where the accent is not on the penultimate eg glanhau, caniatau, (but on the last syll.) the stress will not shift if a one syllable ending is added eg glanhaodd. Cofion, Paul Birt.