how many students are there in the class比如这种句子

那不一样
加there,就是there be句型,there是主语how many students are in the class?完整的回答There are 20 students in the class
不加,就是 具体的名词或代词做主语
how many students are in the class?完整的回答20 students are in the class
可以看出.不加there 不是主系表结构