is,am,are这些都是BE动词，是谓语，它们通常在句子中承担起指示、肯定或者确定某样事务的功能。
is用在第三人称单数上，如He is a handsome boy./She is very pretty.
is的否定形式是is not,即isn't.
过去式是was.
am只用在“我”身上。如I am a student from Yucai Middle School.
am的否定形式是aren't.（这个比较特别）
过去式是were.
are用在第三人称复数上，如They are all Americans.
are的否定形式是aren't.
过去式是were.