A very common occurrence of soft mutations is immediately after any of a large group of prepositions, which are words indicating a relationship between components of the sentence. These are:
am: 'for' , sometimes in the context 'in favour of..'
Dw i'n gofyn am ddiod I am asking for a drink
Rydw i am fynd adre I want to go home
Maen nhw am werthu'r car They want to sell the car
ar: 'on' or 'to'
Mae llwyddiant yn dibynnu ar bawb Success depends on everyone Dwi'n gwrando ar gerddoriaeth I listen to music
at: 'to'
Ysgrifennais at reolwr y cwmni I wrote to the manger of the company
Dwi'n edrych ymlaen at ddod I am looking forward to coming
dan: 'under'
Rhedodd e dan bont He ran under a bridge
Mae'r sefyllfa dan reolaeth The situation is under control
dros: 'over'
Dringodd dros gamfa He climbed over a stile
Cymerodd dros mlwyddyn i gwblhau It took over a year to complete
gan: 'with' , 'from'
Mae'r gwaith gan gyfansoddwr enwog
The work is by a famous composer
Prynais bara gan ddynes yn y pentref
I bought bread from a woman in the village