she is beautiful "她漂亮"， 不能说成 "她是 漂亮" 为什么?

汉语教科书上写着汉语里主词和形容词之间不加 动词" 是"
所以
she is beautiful ， 应该说成;
"她漂亮"， 不能说成 "她是 漂亮"
那学习汉语的外国人学生们就问 那为什么
this is big 可以说成: 这是大的
this car is blue. 不说成 ; 这辆汽车蓝， 而说成: 这辆汽车是蓝的
有什么逻辑，和理论来跟他们解释 吗

She is clever as well as beautiful. She is as plain as her mother is beautiful she is a beautiful girl.哪个是宾语哦?.. she is the most beautiful girl in the class She is a very beautiful giel!(改为同意句) 歌词中有she is a beautiful girl you don*t love a woman because she is beautiful , but she is beautiful because you love her Although she is only 14, she looks like a ____ woman, beautiful anf full in figure. she is as plain as her mother is beautiful高手帮看下这个句子有什么问题 She is as plain as her mother is beautiful.什么意思？