Why is it nous mangeons instead of nous mangons?
… I thought that we drop the ending and add “-ons” to -ER verbs.
You are correct that when conjugating an “ER verb,” the nous form takes an –ons stem added to the root of the word.
So in a normal example using the verb “chanter,” we would say nous chantons (we sing).
However in the case of “manger” we keep the “e” purely for pronunciation purposes.
I want you to say, “je mange,” out loud.
Okay, now try to say, “nous mangons.”
Without the “e,” this sounds like “man-guhon” instead of the characteristic soft g sound: “man-jhon.”
So in French, we keep the “e” in order to maintain the characteristic sound of the verb. Adding the “e” also applies to the vowels a, o, and u.
And manger isn’t the only example!
Look at the French verb changer: Nous changeons.