Learn English – Is “most superior” correct

I think eatable is more often used to mean palatable.

eatable sample usage

Whereas edible is more often used in relation to plants etc that are not poisonous.

edible sample usage

But there is an overlap.

(I learnt the sample usage trick from Anthony Quinn)

"What is wrong with XYZ?" is valid regardless of whether XYZ is one thing or many, and whether the questioner expects an answer detailing one fault or many.

If the questioner wanted to explicitly indicate that he expects an answer listing multiple faults, he'd have to say something like "What things are wrong with XYZ?".

"What are wrong with XYZ?" is never grammatically valid. Nor is it a common mistake — I don't recall seeing it before now, and Google Books records only 14 instances of "what are wrong with", compared to millions of "what is wrong with".

As regards exactly why the non-standard usage is unacceptable, rather than just uncommon, what in this construction is a non-count pronoun. Non-count nouns require the singular verb form.

Edit: Kudos to JLG for highlighting the importance of the word wrong in this construction. The interrogative pronoun what attaches to wrong — a non-count abstract noun which transfers its non-count status to what. That doesn't happen with, for example, "What are those things?".