为什么英语有些句子要用is、are、was,有些就不用呢.例如she is a beautiful girl!

为什么英语有些句子要用is、are、was,有些就不用呢.例如she is a beautiful girl! he was a boy,she was a girl 为什么是用was而 不用is she is a beautiful girl 为什么有些句子要用be的原型呢?不能用am is are来代替,什么情况下必须... what a beautiful girl she is 还是it is为什么? 为什么有些英语句子如Is this your father?要用Yes ,it is.而一些提问要用Yes,she is she is a beautiful girl.分析一下句子的成分? She is a beautiful girl I think. that a girl beautiful she is such a beautiful girl she is!so beautiful the gi you are a good girl中间为什么用are 而不用is? she is beautiful girl 英文歌词