1.I don't know what's wrong.
2.I don't know what is the matter.
3.I don't know what the trouble is.
4.I don't know what the problem is.
原因：宾语要求用陈述句语序,即主语在前,谓语在后.第1、2句中的主语是what,因此不用改变语序.但第3、4句中的what不是主语,the trouble和problem是主语,因此把what放前后,后面的语序应先写主语（the trouble和problem）然后写动词（is）.