It says "the old actor," so why isn't the word "oyuncu" not in the accusative case like "oyuncuyu"? Also, why is using "eski" instead of "yasli" considered wrong?
My understanding of that is that Hotel, is the Object, so is the only one that can have accusative (let's see if I get that one right) :). Also, I'm fairly sure eski for things, Yasmin for people.
correct. subjects cannot be in the dative/accusative/locative/ablative case, they are not objects.
eski is the opposite of new, yaşlı is the opposite of young
eski is used for an old object or thing yaşlı is used for an old person and i think animals. my way of remembering was yaş being age
Would I be right in assuming we can't/shouldn't have the accusitive and the dative? How do I differentiate between 'the hotel' and 'a hotel'? If I wanted it not to be THE hotel, would I just add 'bir'?
Yes. You can only add one case at a time. :)
And you are correct in assuming that you need to add "bir." That is exactly how you can distinguish between the two. :)
otele yaşlı oyuncu geliyor would that not be understood is my sentence structure not acceptable
It is fine. Just a forgotten alternative :)
In that case "yaşlı oyuncu" will not be definite.