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”。