"I'm an actress."
Translation:Actores dw i.
"Yn" is used in many different ways in welsh - one of which is before a verb that's in progress. It is also used as a preposition "in" (eg. "in the room"="yn yr ystafell". Finally it is used before predicative adjectives (as opposed to attributive adjectives). Eg. the difference between "She is happy" and "The happy woman": "mae hi'n hapus" and "y ddynes hapus". "Yn" is used in the former but not the latter.
Not a grammar expert but speak welsh at home - so I may need someone to back me up on the technicalities here :)