The 是。。。的 construction is very interesting and is used to emphasize specific aspects about a sentence. It's mostly used to ask questions that seek specific information, usually in the past. The pattern is: SUBJ + 是 + INFORMATION + VERB + 的 For example: 你是怎麽來的？ 我是坐飛機來的. (Emphasizes that I came by plane) This page has more than enough information on the 是。。。的 pattern; I only gave a basic overview. http://resources.allsetlearning.com/chinese/grammar/ASG8YTSF

你是做什么的? in cases like this, the part after 是 becomes an adjective. 做什么的 is thus used as an adjective. You can omit 是 in most cases, but the grammar behind it stays the same. It sounds weird from the English perspective, it's like English verbs becomes noun or adjective when the -ing suffix is added. For example, I am daunted vs it's daunting. An non-English speaker might ask why it is daunting, why not it daunts. :-), so to sum up when 的 is added, the phrase becomes an adjective. 我是唱歌的 我是坐车来的 我是编程的 我是来工作的， 3 verbs in a row, kinda mind bending, but 来工作的 becomes.... what? an adjective. Hope it helps

学生。

Thanks!

It's the famous 是......的 construction. In this case it emphasises the "what" in the question. It's actually one of the most elementary lessons when studying Chinese.