"I eat tomatoes at breakfast."
Translation:Kahvaltıda domates yerim.
That is locative ("at the..."), whereas in this case "domates" is tied to an ablative construction . Think of nouns in Turkish as metaphysical unless otherwise noted. It is the metaphysical ideal of "tomatoness" until it is placed in a specific time and place by a sentence construction. Hope that made helped!
My impression is, this is similar to the behavior of uncountable nouns in english - except that it's applicable to countable ones - you'd say "I had wine for dinner" - not "one wine" or "wines", right? (I'm not a native English speaker, and not certain: could you say "I had tomato for breakfast" if you want to put emphasis on what you had no matter how much of it?). By the way - is "at breakfast" in the given context the same as "for breakfast"? I think I tried "for breakfast" once and it was counted as an error. Not quite sure though if there wasn't another problem with my answer.