Question

how many subjects are there in your school为什么要加上are there

Answer 1

There are 是一个句型 there be。
There be 句型是英语中常用句型， 意思是“有”，表示“人或事物的存在”或“某地有某物”。

这句话是一个疑问句，你学校有几门课？
疑问句式中 助动词和系动词要放在疑问词后，
所以要用are there

Answer 2

回答这个问句先：There are 3 subjects in my school.
再回来看问题，那个 are there 不就明白了。

Answer 3

因为你回答的时候要说There are xxx subjects in my school.

Answer 4

尼玛呀，旧题呀