Semi-regular -er verbs (2): Orthographically-motivated rules
Consider the nous form of the verb manger. The ending for the nous form is -ons. This would give the expected form *nous mangons. However, the -g- in manger represents a "soft" g sound (like the j), as it does generally before the letter e (and i and y). Whereas before an o (or indeed a or u), it generally represents < "hard" g sound.
To smooth out this inconsistency, an -e- is inserted before any ending beginning with o or a. In the present tense, this means before -ons of the nous form.
So the correct nous form of manger is nous mangeons.
Similarly, the letter c generally represents a "soft" c sound (actually, the same as s) before e, i or y, but not before a, o or u.
So before an ending beginning with o- (or indeed a- or u-), the -c- of the stem is replaced with a c cedilla: ç.
So the correct nous form of commencer is nous commençons.
We consider other types of spelling and pronunciation complications on the following page.