"He eats a cheese sandwich."
Translation:Er isst ein Käsesandwich.
I don't see a problem with either of the sentences. Which word do you mean?
does the gender of a sandwich depend on sandwich or thetype of sandwich it is? weird wording so for example: in Fischsandwich, does Fisch dictate gender or does Sandwich?
The second noun dictates the gender, i.e. in this case "Sandwich". "Sandwich" can be either masculine or neuter, so the whole word is either "das Fischsandwich" (which I would prefer) or "der Fischsandwich". The reason behind the two genders is probably that "Sandwich" is a loan word from English. It takes some time before there is a consensus on gender.