his tutor say language is a tool with which people express their idear thoughts for it中with 起什么作用?

题目

his tutor say language is a tool with which people express their idear thoughts for it中with 起什么作用?

答案

with which意思是“用……,通过……”,也即是人们通过语言这个工具来表达他们心中的想法.
tool是宾语先行词,with which在从句中起到介词短语的作用
要理解这种题,你就把从句当一个完整句子去看待,就可以看出引导词的作用了