帮我分析一下这个句子的结构,后面那个半句没谓语啊

问题描述:

帮我分析一下这个句子的结构,后面那个半句没谓语啊
It'a one thing if a scientific experiment goes wrong in a lab,they say,but something else entirely if it winds up on your dinner plate

It'a one thing if a scientific experiment goes wrong in a lab,they say,but (it is) something else entirely if it winds up on your dinner plate
这句省略了it is嗯,那有相关的语法知识点的讲解么,谢谢...