Final answer:
The irregular verb in the imperfect tense is 'era'.
Step-by-step explanation:
The irregular verb in the imperfect tense can be found in the sentence: Mi madrastra Eloisa era una mujer fuerte y estricta. In this sentence, the verb 'era' is irregular because it does not follow the regular conjugation pattern for -er verbs in the imperfect tense. Instead of the expected form 'era', it should have been 'erais' if it followed the regular pattern.
Learn more about imperfect tense in Spanish