"She is eating bread."
Translation:Elle mange du pain.
because "elle mange le pain/she is eating the bread" means something else.
the nuance is as follows:
she is eating the bread = this bread, the bread on the table, a defined bread => definite article THE/LE
she is eating bread = some bread, a piece of bread, an undefined quantity of bread => partitive DU
In English, to mean that an action is in progress at the time you speak, you use the continuous verbal form, ie verb BE + action verb in the gerund form (-ing). o he is eating means he currently eats
In French, this verbal form does not exist (directly translated “il est mangeant” is incorrect).
Therefore, you can translate either “il mange” or “il est en train de manger”, where the construction verb être + en train de + infinitive correctly expresses the English continuous form.