lm的完整形式：I am , I 是主语，如果am 后面跟名词或者形容词,它们都是系动词.如果跟动词ing 形式,它们都是助动词.

例如：

I am a teacher .

He is a student .

They are happy .

例如：

She is singing .

I‘m flying a kite .

They are playing football