is用在第三人稱單數上，如He is a handsome boy./She is very pretty.
is的否定形式是is not,即isn't.
過去式是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.（這個比較特別）
過去式是were.
are用在第三人稱復數上，如They are all Americans.
are的否定形式是aren't.