I don't know what do you say.

題目：

I don't know what do you say.
爲什麼what後面要加do?
I don't know what you say.不可以嗎.

解答：

你的說法是正確的,應該是I don't know what you say.
這句話是由連接詞what引導的賓語從句.
賓語從句所表達的意思無論是陳述還是疑問,用作賓語從句時必須是陳述句語序,也就是說主謂次序不能顛倒.
再舉幾個例子：
I don't know what you want to eat.我不知道你想吃什麼.
I don't know where he comes from.我不知道他是哪裡人.
I don't know when the bus will arrive.我不知道汽車什麼時候到.
上述幾個例句的賓語從句,雖然表達的都是疑問,但都是陳述句語序,而不是疑問句語序.