Answer: Trabajaba (imperfect)
Yo trabajaba mucho en la tienda todos los días.
I worked a lot in the store every day.
Step-by-step explanation: To describe in Spanish a habitual behavior in the past we have to use the imperfect past. As the subject in this sentence is "Yo", which is the first-person in the singular, we have to conjugate the verb in this form. Yo trabajaba.