Final answer:
The imperfect form of the verb 'ser' in the sentence is correct and matches with the subject and context.
Step-by-step explanation:
The sentence Éramos una hermosa tarde de verano is correct. The imperfect form of the verb 'ser' (to be) in this sentence, 'éramos', is correctly used to describe a past ongoing action in the first person plural (we). The verb form matches with the subject and the context of the sentence, indicating that they were a beautiful summer afternoon.
Learn more about Imperfect verb form usage