I thinkyou are a good girl〔改为否定句〕

I think you aren't a good girl.
一般在be动词后面加not 也可以在主语后加don't!!!

I don't think you are a good girl 才是正确的

think 的否定时，一般都是助动词do/does 后面加上not ，而不是在be 动词后加 not~

I don't think you are a good girl

在带宾语从句的复合句中，如果主句的谓语动词是think, believe, suppose, expect, guess, fancy, feel, imagine等表示“观点、信念、推测”等心理活动的动词时，否定谓语从句的not往往转移到主句，成为“形式否定主句，意义否定从句”，这种语法现象就叫做否定转移