There was messy 为什么会错呢?为什么There was mess才对呢?我是五年级的学生.

there be 表示.有.
所以后面接的词应该是名词
而messy是个形容词,当然不可以单独接在there be后面.
再问： 老师说be动词表示是的意思啊、、、、可以回答更能让我明白么？
再答： 这是一个there be 句型表示的是 “某处/时间有(存在)某人或某物”，其结构为There be(is，are，was, were )+名词+地点状语/时间状语。例如： There are fifty-two students in our class. There is a pencil in my pencil-case. There was an old house by the river five years ago.