In my another post ("only has" vs. "has only", when to use which?) I said
This post is not a duplicate of ..., since that post does not explain why could "only" not be used as adjective in my 2nd example.
"why could sth not be used as ...", is it an idiomatic expression?
"Explain why could 'only' not be used" is not grammatically correct and it's not idiomatic.
Here's what the Macmillan Dictionary says about why:
Why can be used in the following ways:
as a question adverb (introducing a direct or indirect question): Why didn’t you call me?I know why he’s worried. (followed by an infinitive without “to”): We can start now, so why wait?
as a relative adverb (referring back to a noun and introducing a relative clause): She’s tired and that’s the reason why she’s so bad-tempered. (introducing a clause that is the subject, object, or complement of another clause): Everyone in Dublin is so friendly, and that is why I love the city.
https://www.macmillandictionary.com/us/dictionary/american/why_1
When why is used as a relative adverb - as it is in your sentence - it must be followed by a noun or a pronoun:
Tell me why he did it
Explain why dogs chase cats
This is true even when the sentence has an auxiliary verb:
Tell me why he had done it
Explain why dogs were chasing cats
This is different from sentences that use why as a question adverb:
Why had he done it?
Why were dogs chasing cats?
In your sentence, you seem to be reversing the standard word order, as if this sentence were a question, but it's not. You're using why as a relative adverb, so the standard word order must be used:
That post does not explain why "only" could not be used as an adjective in my second example.
