Depends on the context.
A calf eats bread. General statement about calves and bread.
The calf eats bread. Statement about a specific calf's eating habit.
A calf eats the bread. An unidentified calf eats a specific piece of bread.
The calf eats the bread. A specific calf eats a specific piece of bread.
All of these are perfectly natural in English. Not sure of the Romanian. Perhaps it requires the definite article for "bread" in every case above. Many languages do.
I actually translated it as "A calf is eating the bread." and that was accepted as correct.