I am a beautiful girl.改为一般疑问句是:Are you a beautiful girl?为什么不是:am I a beautiful girl?
答:当我们把陈述句变为一般疑问句时,习惯上要把第一人称I变为第二人称you。(当然,如果不改变人称,也算正确。即:AmI a beautiful girl?)即:
Are you a (beautiful)girl?