As the verb “knew” is in the past tense, the verb “was” must be in the past. On the other hand, there is an indeterminate pronoun, none that can be either singular or plural; It doesn`t matter if you use a singular or a plural plate, unless something else in the sentence determines its number. “A prepositional sentence cannot contain the subject of the sentence. Don`t be confused if a prepositional sentence (a sentence starting with, between the two, between the two, etc.) is between the subject and the verb. . . .