In Japanese, negative and past tense are all expressed by conjugation. We can conjugate a noun or adjective to either its negative or past tense to say that something is not [X] or that something was [X]. This may be a bit hard to grasp at first but none of these state-of-being conjugations make anything declarative like 「だ」 does. We'll learn how to make these tenses declarative by attaching 「だ」 to the end of the sentence in a later lesson.