"He is a professional actor."
Translation:Hij is een professionele acteur.
Do I have to use "een" here? I didn't use it and it was marked wrong. I kept putting "een" with other job related sentences on here and it gave me another translation without the "een". It's because I kept forgetting that you often don't use an article if you are talking about someone's proffession. But in this case there is an adjective involved so I'm wondering if that has something to do with it...
It is correct, but probably confusing to remember as such, because it is not a standard situation. Leaving off the -e is used only when we describe the professional aspect of someone which does not necessarily reflect on the person. Thus, "Churchill was een groot politicus" means he was important as a politician, not that he was tall. See https://onzetaal.nl/taaladvies/advies/een-groot-grote-man