be不能单独作谓语动词； 它既是系动词也是助动词，如 He is a student .系动词He is very strong .系动词He is studying English .助动词English is taught in their school . 助动词作系动词时后接名词，形容词或副词作助动词时与其他动词一起构成一种时态或语态，如现在进行，过去进行时；被动语态
