There does not seem to be a consistent pattern for translating plurals. For instance in this sentence "The man has the plates." is correct, but "The man has plates." is marked incorrect. Other places the more natural English translation of plural without the article is accepted. Is this deliberate, or just beta refinements in the making?
The problem as I see it is not the plural but that the Eng. without the article was not accepted. It should have been because as we know in Greek we use the article much more so in English it could also be without the article. I've added it as an alternative. Yes, we continue to refine the course and your input is appreciated.
Yes, I agree; it is not particularly an issue with plurals, but with articles. These just happened to be two cases involving plurals. However, on the other hand, in English, we are much quicker to drop the article in sentences involving plurals than in sentences involving singulars. For instance, we would never say "Bird is singing", instead of "the bird is singing", but do say either "Birds are singing" or "The birds are singing"
Πιάτο (πιάτα in plural) is a neuter noun. The neuter article in plural is τα, in both the nominative and the accusative (the latter is used here). You can review article declensions at https://www.duolingo.com/skill/el/Basics-1/tips-and-notes.
Εφημερίδα (pl εφημερίδες) is a feminine noun, therefore uses the definite article forms οι/των/τις in plural.
The accusative is used because "The man has them", not they. :)
This is explained in the comment just above yours.
Also, check out this site where you can type a word in any form and see the various ways it is used: