Is this "why" an interrogative or a relative adverb? They say it's both interrogative and relative adverb. Is it correct? What is the standard to differentiate the two?
In which cases, is it interrogative and in which cases, it is relative adverb?
ex)It is easy to see why marine scientists spend their lives studying the humpback whale.
Nobody knows why these whales sing like troubadours
I wouldn't say "the reason" is omitted. You could say, "Nobody knows the reason [that] these whales sing...."
"What is the reason whales sing?"; "Why do whales sing?" "the reason" is not omitted from the second sentence - it just isn't there.
I would not say or write "the reason why", as explained in another recent thread (that discusses "the place where", "the time when", "the manner how" ...)
I would also not say or write, "What is the reason why whales sing?"
"Do you know why whales sing?" - "Why" is a relative adverb. I can see your point now about omitting "the reason". You want the relative adverb to relate to something. In that case, yes, you can infer "the reason", but you don't need to say it. "Why?" = "What is the reason?"
Then, if you transfrom this sentence to "Why do marine scientists spend their lives..?", then is it interrogative in a direct question? And in any indirect questions, it becomes a relative pronoun?
Some grammar material says if (why,where,when) is connected with questionable verbs such as "ask, know, curious", it becomes interrogative, otherwise relative adverb.
But your standard seems to depend on direct or indirect questions, that's why I'm confused. Can you understand my point?
As I suggested in the other thread - When these words introduce a direct question, they are interrogative. If they don't,they aren't. 'Interrogative' means 'asking a question/in question-form'. Indirect questions do not directly ask a quastion; they are nor expressed in a question form, and are not interrogative.
We are in a bit of a circular discussion here. A relative cannot be an interrogative, because it is not interrogative.