Translation:A television programme
What triggers the soft mutation? I was expecting "teledu". Does the use of an adverb, even without "yn", also cause a soft mutation?
Just a hint on grammatical gender. "Rhaglen" ends in "-en" which is commonly found at the end of feminine nouns e.g "Coeden", "Lloeren" and "Cneuen" (Tree, Satellite and Nut). The masculine 'version' is "-yn" e.g "Plentyn", "Bachyn" and "Twpsyn" (Child, Hook and Idiot).
Adjectives mutate after feminine nouns. teledu here is acting as an adjective. rhaglen is a feminine noun. And so...
And yes, many adverbs are ready-mutated: ddoe, weithiau, ledled, bob tro, fesul un, ... (yesterday, sometimes, all over, every time, one by one, ...)