Realizing that this will be of no help whatsoever, the subject is what.-faldage

Cantankerousness is the "what" that is the matter with me. -a paraphrase of the words of tsuwm

Dear soojin,

Every living fiber of my being wants to agree with the good Doc Bill, with Jackie, with Mr. Troves, and with the wonderful Missus of troy. They are nice people, almost like kinfolk, but on the matter of this subject their observations are about as useless as teats on a boar hog.

Instead I am forced to side with the likes of tsuwm and Mister Faldage.

When two entities are connected by "is", the subject of the sentence is indeterminable, and if one chooses, he chooses arbitrarily. "Is", you see, equals "equal". And no, Monica, it doesn't depend on what Bill meant by "is".

A) What is the matter with you?
B) What is the subject of (A)?

soojin,
Just remove the question mark, then (B) becomes the answer to (A) and "what" is also the answer to (B)

Interchanging the nouns around "is" is only a feel-good method to make you think that you have thought.

Forget "interrogative pronouns" unless you are taking a grammar test. Then forget them quickly after, if you are.

Good Luck.