1、句子中主语为I(也就是“我”)时,就要用到am;比如, I am a girl .
2、句子中主语为he,she,it(也就是“他,她,它”)时,就要用到is;比如,She is cooking.比如,It is a snake.
3、句子中主语是you的时候,要用到are;比如,You are a good man.
4、句子中主语是they或者是we的时候,要用到are;比如,We are family.
5、句子中主语为名词复数时,也要用到are;比如,The girls are singing.