We work in a big factory.
讲述“谁”：我们
谓语：work
宾语：in a big factory
The classroom is very big.
讲述“什么”：教室
谓语：is
表语：very big
Three are enough.
数词作主语：Three
谓语：are
What we need is food.
从句作主语：What we need
谓语：is
表语：food
I sing a song at home.
主语：I
谓语：sing
地点状语：at home
这些句子都是简单句，主语分别是“We”、“The classroom”、“Three”、“What we need”和“I”，谓语动词分别是“work”、“is”、“are”、“is”和“sing”，宾语或表语分别是“in a big factory”、“very big”、“enough”、“food”和“at home”。