am与第一人称一起用,如: I am a doctor. I am young. is与第三人称一起用:如:He is a doctor.She is young.The desk is old. was是is的过去时形式,在过去时中用。如:He was here yesterday.他昨天在这。 are与除以上几个人称以外的词一起用,如:你,你们或者复数的主语一起用。如:you are a doctor.We are young. The books are old. be是原形,用在助动词或其它词后面。如:you should be careful next time. I want to be a doctor when I grow up.