Why Does the Format "_____ Is" When Asking a Question Sound Correct in Some Situations, but Not in Others

I'm not a native speaker, but I will give you my view.

In copulative sentences, that is, in sentences where the main verb is "be", subject and subject complement (of a nominal nature) are usually interchangeable:

1.a. Mary is my best friend.

1.b. My best friend is Mary.

In your first sentence, the main verb is "go", so there is no doubt that "she" is the subject and there will be no problem placing it before the auxiliary in a short answer:

• Who is going to the game tonight?
• Mary (is).

Your second and third sentences are copulative and have "be" as the main verb. "what" and "who" may then ask for the subject or for the subject complement. If the information requested is the subject, then the subject + auxiliary answer will sound fine, but if the information requested is the subject complement, it will sound odd.

Of these two pairs of sentences, we can easily tell that the one that sounds natural is that which introduces new information by means of the subject complement:

2.a. Football is the most popular sport in America.

2.b. The most popular sport in America is football.

3.a. The name of this book is The Catcher in the Rye.

3.b. The Catcher in the Rye is the name of this book.

2.a. and 2.b. are equally natural. Instead, 3.a. sounds more natural than 3.b., and that is because "the name of this book" is a better candidate for subject than "The Catcher in the Rye". Then, when we ask:

• What is the name of this book?

"what" is asking for the subject complement. A more natural reply will then be:

• It (The name of this book)'s The Catcher in the Rye.

I’ve come up with some sort of rule to identify whether the wh-word asks for the subject or for the subject complement, which consists of checking the position the reply to "what”, “who” or “which” will occupy in the answer to the question. Perhaps, as Cascabel said, this belongs on ELL, but since the question has been posed on ELU and has not been migrated (at least, not for the time being), I’d appreciate your comments:

Q2: What is the most popular sport in America?

A2: Football is the most popular sport in America.

“what” may be considered to be in subject position, because it can be replaced with “football” in that front position in the answer. This accounts for the correctness of the short answer “Football is,” and also for the order of the words if the question is embedded:

Embedded Q2: He wants to know what is the most popular sport in America. (what: subject / is: verb / the most popular sport in America: subject complement)

Alternatively, we can consider that "what" (and "football") is in subject complement position:

Q2': What is the most popular sport in America?

A2': The most popular sport in America is football.

In this case, the embedded question will be different:

Embedded Q2': He wants to know what the most popular sport in America is. (what: subject complement / the most popular sport in America: subject complement / is: verb)

Let’s see what happens with the other question:

Q3: What is the name of this book?

A3: (It’s) The Catcher in the Rye.

In this case, “what” is in subject complement position, and that accounts for the awkwardness of the answer “The Catcher in the Rye is.” To further prove this point, we can check what happens when the question is embedded:

Embedded Q3: He wants to know what the name of this book is. (I don't think He wants to know what is the name of this book is a good reported question.) (Therefore: what: subject complement / the name of this book: subject / is: verb)

(I admit that other indirect questions would sound more natural, like “He wants to know the name/title of this book” or “He wants to know what this book is called” where, again, “this book” is subject and “what” asks for the subject complement.)

Word proximity habit

Sometimes correct grammar sounds wrong because of our habits. Word proximity is one of the greater factors in forming our habits. I like to call this habit "auto-preference".

(Maybe its from bad habits, maybe unsophisticated habits, or maybe it is just a less common usage we've encountered.)

In the examples you gave, all correct and IMHO preferred, the responses you liked each had one word before "is". The response you didn't like had five words before "is". This goes against our habit if having very few words before "is".

Good examples of "strange sounding" use is with possessive pronouns and matching number with the verb.

Answering the phone to "Is Susan there?"

What we often expect:

This is her.

More proper:

This is she.

...Technically both are correct, but the second one sounds strange because we are accustomed to using object pronouns immediately after the verb. In this case, the first is correct because it treats "her" as an object in the predicate. The second is correct because we use the subject form for a predicate nominative. Either may be used.

Consider object pronouns

What we often expect:

The boss called Mitchell and I.

Correct:

The boss called Mitchell and me.

The issue here is that our "auto-preference" likes to use object pronouns just after verbs, not later in a multi-noun object.

Consider matching the verb with a singular noun having a plural noun decribing it...

What we might expect:

Jonathan of the Kansas City Royals eat popcorn.

Correct:

Jonathan of the Kansas City Royals eats popcorn.

...That one may seem strange to some of us because, most of the time, the subject of the verb comes before it, so our "auto-preference" tries to match the verb with the word before it rather than with the actual subject of the verb. While this may not seem strange to everyone, make it longer and more complex and it will seem strange to more people.

Try this from the thread, which even seemed strange to me. It is correct because the verb and subject must match...

What our habits might wrongly expect:

anything that works against our habits seem strange.

Correct:

anything that works against our habits seems strange.

...Again, having this habitual preference because of "proximity". Our "auto-preference" wants patterns among words right next to each other.

Further consideration

On a teaching note, this could be an argument for why memorizing literature or repeatedly practicing correct phrases could prove useful in learning English, native or ESL.

Isn't it just because, in the response, "Catcher in the Rye" refers to the title of an entity, not the entity itself? That's why you can say:

—Which book is left out of the list?

—Catcher in the Rye is.

Here, "Catcher in the Rye" refers to the book as an entity, not to its title.

You could say:

—What's your favorite book?

—Catcher in the Rye is.

This somewhat implies that the book is known. "Catcher in the Rye" refers to the book, not the title. If the responder assumes that the asker hasn't heard of the book, the response would probably be just "Catcher in the Rye." This would be akin to saying "It's called Catcher in the Rye."