Word 'mange '
Mange means eat/am eating. Now when this word is used in some French sentence, how to know, how to know which tense is being referred to, here?
“Je mange” can mean “I eat”, “I am eating” or “I do eat”. It is through context that you know which sense is intended. Duolingo should accept both “eat” or “am eating” in the translation exercises.
I forgot about "I do eat" - well pointed out. This would be a rather emphatic response to someone who, perhaps, accuses you of not eating (maybe being anorexic). For example:
"Tu ne mange jamais rien !" → "You never eat anything!"
to which the response might, indignantly (or defensively) be:
"Si, je mange !" → "Yes, I do eat!"
A word of advice, use the web version of duolingo and read the tips notes for every lesson. You will find your way a lot easier than via the apps.
Manger means "to eat". When conjugated in the present, it is this:
As you can see, "mange" is used in the present tense with "je" and "il/elle/on". However, this changes with different tenses. I would suggest going to wordreference.com and looking at all of the conjugation forms of "manger" in each tense. ^ ^
You can tell by the context, most languages are like this except for English. If you are talking and you really want to refer to "I am eating" you can use "Je suis en train de manger ..." which would mean I am in the middle of eating.