was
The conjugation of the "to be" verb in present tense is "am, is, are". In past tense it's "was, were". First person pronouns are "I, me, my, we, us". "We" and "us" are plural because they include "I" and "You" whereas "I" is singular. Therefore, the correct conjugation for "to be" in past tense is "was" since it is the correct past tense verb to use with "I": I was late yesterday.
