The definite article (the) in Romanian is always behind the noun (măr = apple, mărul = the apple), never before. And it never has any influence on the conjugation scheme used.
I agree with Potestasity that the OP (HollyGenevieve_C) probably was asking why the the third person present of “to eat” is mănâncă if the infinitive is a mânca. Potestasity is clearly speaking about an irregular extra syllable. I repeat, this is the normal, though irregular form of the present of a mânca in singular and in the third person plural. And it has nothing to do with articles, demonstratives or similar.
I am getting a lot of negative comments in the last time, so there's no reason you should believe my word but you can check the form at conjugarea.ro or at the English Wiktionary. Or under the link provided by Potestasity.
- eu mănânc (I eat)
- tu mănânci (you eat)
- ea/el mănâncă (she/he eats)
- noi mâncam (we eat)
- voi mâncați (you – pl – eat)
- ele/ei mănâncă (they eat)
Last year LucasDiniz22 asked about PC, and I answered positively. I still think PC (she is eating the apple) is acceptable and often is the better translation. “Mărul”, however, is the apple, not an apple (un măr).
I can see your point about Present Simple vs Present Continuous but I am not convinced that “she eats the apple” (DL's primary translation) could not be correct in certain situations. Maybe “What does she do when offered an apple and a pear? She eats the apple.”
“She eats apples” would, in my opinion, be translated by “ea mănâncă mere”.
Actually, "She eats the apple" is the correct one from the ones that you mentioned. "Mărul" literally translates to "the apple", and "măr" is simply "apple". "Apples" would be translated to "mere", and "an apple" - "un măr" (probably taught somewhere in the following lessons)