Question

1. nous____ un sandwich en ville. (manger) (1 point)

Answer 1

Any regular -er verb in french is conjugated with endings like these
Drop the er from manger and add any of the endings that match the personal pronoun
Je ....e
Tu .....es
Il/Elle/On ...e
Nous ...ons
Vous ...ez
Ils/Elles ...ent

Answer 2

Yep, with slight orthographic exceptions for verbs such as 'manger' or 'ranger', that will lose the /ʒ/ sound if nothing is done before a, o and u.

Answer 3

Alors
Nous mangeons un sandwich en ville.
Because mangons sounds weird, too :P Without the E you lose the "je"-like sound