Final answer:
The verb needed in the imperfect to complete the sentence 'diez personas en la fiesta' is 'Había'.
Step-by-step explanation:
The verb needed in the imperfect to complete the sentence 'diez personas en la fiesta' is Había. This is because 'Había' is the third person singular form of the imperfect tense of the verb 'haber', which means 'there were' in English. The imperfect tense is used to describe ongoing or repeated actions in the past. In this case, it indicates that there were ten people at the party.
Learn more about Imperfect verb in Spanish