be 是am is are 的原形,主要用在不定式to,情态动词等的后面,例如,That cannot be my headmaster,because he has gong to London.
What I want to be doctor in the future.
am是当主语是I.is 是当主语是第三人称单数时.are 是当主语是第一及其第三人称的复数、第二人称时用.例如I am a student.
She is a teacher.We are all stidents.They are workers.You are watching TV.