I think it neccessary for students to take an interest in al

这里是形式宾语,因为it后面没有谓语动词,所以it只是做think的宾语
再如
I think it is neccessary for students to take an interest in all subjects .
这里it 后面加了一个is ,此时it做的就是形式主语,相当于think后面省略了that,这是that引导宾语从句,而it做宾语从句的形式主语.
判断it做形式主语还是形式宾语,主要看 it后面是否是一个完整的句子,也就是后面是否有谓语动词,如果有就是形式主语,如果没有就是形式宾语

首先“必须”这个单词多了个"c"，这个句子写法没有问题。
这个句中 it 做形式宾语，真正宾语是动词不定式to do sth, 所以同意句是：
I think that to take an interest in all subjects for students is necessary.
也可说
I think that to take an interest...

首先，neccessary 多了个C
再次，你这句子是错误的，少个IS

I think (that) it's necessary for students to make an interest in all subjects it还是作后面宾语从句的形式主语希望对你有帮助~ I think __能再详细些吗？I think it neccessary for students to take an interest in all subjects .这句话...