I don't know what's the matter.还是I don't know what the matter was.?

按语法规则来说，I don't know what the matter was是对的
可是在与老外对话时，一般都说I don't know what's the matter.

I don't know what's the matter. 是对的，what在句中作主语。

祝你进步！

我们以前学语法的时候,这句话的What是做表语的.

I don't know what the matter was.是对的.

I don't know what the matter was.

宾语从句中的疑问句是要用正常语序，而不是倒装，例如I don't know what this is. 主语从句中也一样，所以what this is is unclear在语法上才是正确的。不过话说回来，这样似乎并不是地道的英语，可能说It is unclear what this is 或者I don't know what this is会更好 。

Ls的对

i don't know what the matter is