Question

I don't know what do you say.

Answer 1

你的说法是正确的,应该是I don't know what you say.

这句话是由连接词what引导的宾语从句.

宾语从句所表达的意思无论是陈述还是疑问,用作宾语从句时必须是陈述句语序,也就是说主谓次序不能颠倒.

再举几个例子：

I don't know what you want to eat.我不知道你想吃什么.

I don't know where he comes from.我不知道他是哪里人.

I don't know when the bus will arrive.我不知道汽车什么时候到.

上述几个例句的宾语从句,虽然表达的都是疑问,但都是陈述句语序,而不是疑问句语序.