You've already met the -ger and -cer spelling rules in our -ER verbs lesson. The imparfait needs them too, but only in some forms, which trips up even confident students.
1. -ger verbs: an extra e before a/o
Manger and other -ger verbs add an extra e whenever the ending starts with a, to keep the g soft.
| je mangeais, tu mangeais, il mangeait, ils mangeaient | I/you/he/they was/were eating |
But not before -ions/-iez: i already keeps the g soft on its own, so nous and vous drop the extra e: nous mangions (not "mangeions"), vous mangiez.
2. -cer verbs: a cedilla before a/o
Commencer and other -cer verbs add a cedilla (ç) in the same forms, for the same reason, keeping the c soft.
| je commençais, tu commençais, il commençait, ils commençaient | I/you/he/they was/were starting |
Same exception: nous commencions, vous commenciez, no cedilla needed, c is already soft before i.
3. Conjugation: manger & commencer
| Pronoun | Manger | Commencer |
|---|---|---|
| je | mangeais | commençais |
| tu | mangeais | commençais |
| il / elle / on | mangeait | commençait |
| nous | mangions | commencions |
| vous | mangiez | commenciez |
| ils / elles | mangeaient | commençaient |