為什麼圖書館的開放時間牌的close 用了過去式。

hi, it is not the 過去式, it is the passive voice in this sentence, because {library} is a {no-life --- death} things, not the same as He, She, Dog ...
in this sentence {is closed} is a passive voice verb, they used together {verb to be, e.g. is, are, be} and {perfect tense, e.g. eaten, went, taken ...) to describe the {death} thing - library.
e.g.
Mary eats an apple. (this is active voice sentence)
An apple is eaten by Mary. (this is passive voice sentence)

你好。

這句子不是用了過去式, 而是選用了 "被動式 Passive Voice" --- is closed, 因圖書館的門在浮動假期是會被人關上的, 而門是不會自己去關閉的。

謝謝。

不是過去式
是被動式, passive voice