I have just come across this textbook on English Language and Art. There appears the sentence
Byline tells you who is the author of an article.
As I remember, we all accept that the correct sentence must be
Byline tells you who the author of an article is.
Can you please explain?
You are broadly right. However, the thing in the box isn't a sentence. It is a dictionary definition.
In a definition, you first have the the headword. It is often printed in bold. There is then a phrase to explain the meaning of the headword. This phrase is often a fragment. So the headword is "Byline", and the definition is "tells you who is the author of the article".
You are right that, that fragment would be better expressed as "tells you who the author of the article is". The expression in the book is okay as a fragment, but would be wrong if you expanded this fragment to a full sentence.
— Who is the author of the article?
— The author of the article is Allan Allandale.
The answer tells us who the author of the article is.
— Who is the author of the article?
— Allan Allandale is the author of the article.
The answer tells us who is the author of the article.
“The author is X” and “X is the author” have approximately the same meaning. They have different emphasis, and in some cases only one is appropriate. But there are many cases where both are possible.
In the relative clause, the normal word order is:
(There can be additional complications but they don't apply here.)
In “who is the author of the article”, the subject is “who”, so the verb comes immediately after it. After that, there is one complement (a predicate).
In “who the author of the article is”, the subject is “the author of the article”, so that comes before the verb. The pronoun “who” is a predicate. It's the only complement of the verb “is” so the clause ends with “is”.
In this context, in a direct clause, it is more idiomatic in this context to say “the author of the article is Allan Allandale”, because we want to express information about the author of the article. It is natural to make “the author of the article” the subject of the sentence.
This is still true in a relative clause. However, in a relative clause, it is a little awkward to have a long fragment between the pronoun and the verb. This compensates the default tendency to make “the author of the article” the subject of the sentence. Hence the sentence “… who is the author of the article” is about as idiomatic as “… who the author of the article is”. The meaning is the same, after all.
With a shorter phrase, there would be a more marked preference for using the same subject as with direct clause. “The byline tells you who the author is” is more idiomatic than “the byline tells you who is the author”. The second sentence isn't grammatically wrong, but it's a bit awkward.
At the risk of upsetting the community: it is a common misconception that there are statutes governing English grammar, or some regulating authority which is mandated to determine what is "correct".
That is far from being the case.
The best we have is conventions, familiarity due to common-usage, and occasionally taboos.
So while "who is the author" is jarring, and would probably be avoided by any author or editor with any veneer of sophistication, it is inappropriate to refer to "officially accepted".
Now it might be that certain words or phrases are not acceptable in- for example- a court, or should be avoided because of their ambiguity, or would cause a well-schooled reader to look carefully at their context. I would suggest that the word "sanctioned" falls into that category, particularly if the context is a letter from a layman to a lawyer.
The context of the current example is an informal "scrapbook-style" guide, typeset in something close to the much-derided Comic Sans, and probably intended for teenagers. The advice might or might not be good, but don't let it bother you.
