be是系动词.
它一般跟在主语后面,起修饰作用.
主语+be动词+形容词/名词.
eg:I am beautiful.

She was a teacher two yers ago.

象 He has been to Beijing.
It was cleaned by my sister.
这些我觉得都凭语感,不必特意去记,到最后你反而会乱,平时多看点英语书集(如书虫),还要多读英语.
好了,祝你成功!^-^

be到底有没有过去式

没有，它是根据人称和时态变化时才有过去式的。