Final answer:
The appropriate imperfect verb to fill in the blank is 'me cepillaba'
Step-by-step explanation:
The appropriate imperfect verb to fill in the blank in the sentence would be me cepillaba. In Spanish, the imperfect tense is used to describe ongoing or habitual actions in the past. In this sentence, the subject (yo) was brushing their teeth (me cepillaba) when the water went out (el agua se corto).
Learn more about The use of imperfect verb in Spanish sentence