What it is?对 还是What is it ?对

作为单句应是What is it?
若放在宾语从句中，应用What it is？如：I want to know what it is.

what is it?意思是这是什么
what it is是不能单独当成一个句子使用的

可是放在从句中
比如i don't know what it is 我不知道这是什么

What is it？是正确的。
这个句子是特殊疑问句，系动词要提前。
像：Who are you？是一样的。

What it is? 不能单独用！而What is it ? 是一个句子！

What is it?
或者
I don't know what it is！

"What it is" is incomplete. You are expected to finish the sentence.
"What is it?" is a complete sentence.