rhaglen deledu, what triggers soft mutation?
I was expecting teledu, why mutate?
That's a very good question.
There are quite a number of scenarios where a soft mutation occurs.
In this example it's because 'rhaglen' is a feminine noun and 'teledu' is acting as an adjective, describing the noun. Adjectives mutute after feminine nouns.
In this course the main triggers are after prepositions and the short forms of the verb in the future and past.
Here's a fuller list:- http://www.bbc.co.uk/wales/learnwelsh/pdf/welshgrammar_mutations.pdf