Not always. Actually YN represents several different words, each of which has a different effect on the word that follows. 1. YN meaning "in", used only with definite nouns, causes nasal mutation: yn Nolgellau = in Dolgellau yn Nhal-y-bont = in Tal-y-bont ym Mangor = in Bangor ym Mhwllheli = in Pwllheli yng Ngwynedd = in Gwynedd yng Nghonwy = in Conwy 2. YN as the particle used to link the verb BOD with a verbnoun, causes no mutation: Dw i'n mynd = I am going Mae'r gath yn bwyta = The cat is eating Roedd y bachgen yn darllen = The boy was reading Bydd fy mam yn garddio = My mother will be gardening 3. YN as a particle used to link the verb BOD with a noun or adjective, and to form adverbs from adjectives, causes soft mutation; however, it does not mutate words beginning with LL or RH: Mae dy wyneb yn frwnt (< brwnt) = Your face is dirty Mae'r cinio'n barod (< parod) = The dinner is ready Mae'r tywydd yn arw (< garw) = The weather is rough Roedd ei feibion yn ganwyr (< canwyr) = His sons were singers Dw i wedi cysgu'n dda (< da) = I have slept well Mae'r ferch honno yn dal (< tal) = That girl is tall Mae'r mêl yn felys (< melys) = Honey is sweet BUT: Mae'r botel yn llawn = The bottle is full Roedd yr anrheg yn rhad = The present was cheap