This is a tough one! Romania definitely has a definite article. Many, in fact. In this case, the definite article is the second 'i' in the word "roșii"The challenge is that the definite article is built into the word. Sometimes as a suffix, sometimes just as a vowel shift. Let's take "girl" for example: fată means girl. But shift the final ă to a and you get fata, which means THE girl. In the plural ("girls") you'll get a vowel shift within the word (a to e, fată to fete) and then the vowel shift at the end of the word (ă to e) for the plural INdefinate ending. So girls is fete, but THE girls is "fetele". There, the definite article is the suffix "-le". So, that's how the definite article works on that one word. But that's a feminine word, and remember there's also masculine and neuter to contend with, and those are different. Plus, not all feminine words work exactly like fată. But before you give up on Romanian (and please don't!), Romanians don't expect you to get the articles correct, and oddly enough definite articles are probably the hardest thing about Romanian. By contrast, you could probably learn their entire subjunctive an an afternoon.
oh, I misunderstood! I think, but I'm not sure, that if you wanted to be specific (definite, as in THE) within a prepositional phrase, you might use "the apple is between THOSE tomatoes" etc. But you're right that after a preposition Romanian generally uses the indefinite (nehotarat) form.