Why is the verb in 3rd position?

Question

In conversation with 2 native German speakers I said:

Meine Mutter sah betrübt und besorgt aus, und sogar hatte meine Tante einen Ausdruck, als ob sie zu denken schien, dass ich vielleicht zu weit gegangen war.

and they corrected me to:

Meine Mutter sah betrübt und besorgt aus, und sogar meine Tante hatte einen Ausdruck, als ob sie zu denken schien, dass ich vielleicht zu weit gegangen war.

Their correction appears to me to put the verb in the 3rd position in the second main sentence, after an adverb (or modal/focus particle) and the subject. I do not understand how this could be grammatical, although Hammer's Grammar does provide several similar examples without justification.

A different, but related question, is discussed in Does the subject always follow the verb when not in first position?. But that discussion does not tell me why putting what I see is a focus particle, sogar, first and the verb second is incorrect.

Why is my sentence incorrect?

Answer 1

The verb is still in second position.

Sogar is a particle which modifies the subsequent part of the sentence. So it should be regarded as

[Sogar meine Tante] [hatte] [einen Ausdruck].

This is the only position that sogar can be in, if it should modify the subject. You can check this, by rearranging the sentence (although I admit, this is no help for a non native-speaker):

[Einen Ausdruck] [hatte] [sogar meine Tante].

As pointed out in the comments by mach, similar words are auch and nur, i.e.

[Nur meine Tante] [hatte] [einen Ausdruck]
[Auch meine Tante] [hatte] [einen Ausdruck]

Finally, you can also modify einen Ausdruck, which then becomes

[Meine Tante] [hatte] [sogar einen Ausdruck]

Answer 2

In my opinion it is not absolutely false, but it sounds "bumpy".

The phrase "Meine Mutter sah betrübt und besorgt aus, und sogar hatte meine Tante einen Ausdruck" consists of two main clauses (Hauptsätze). The first part "Meine Mutter sah betrübt und besorgt aus" is correct, but the problem is the second part

(und) sogar hatte meine Tante einen Ausdruck

Although German word order is variable, everybody would expect

(und) sogar meine Tante hatte einen Ausdruck

in SPO structure. However, the problem is related to the word sogar. If we replace it by the adverb heute, we get

Meine Mutter sah (heute) betrübt und besorgt aus, und heute hatte meine Tante einen Ausdruck

and I can't see anything wrong with it. In fact,

Meine Mutter sah (heute) betrübt und besorgt aus, und heute meine Tante hatte einen Ausdruck

is definitely false.

An alternative (and indisputably correct) formulation of your sentence is

Meine Mutter sah betrübt und besorgt aus, und es hatte sogar meine Tante einen Ausdruck

Update:

"Sogar" is a grammatical particle. Quotation from English Wikipedia:

In grammar, the term particle (abbreviated PTCL) has a traditional meaning, as a part of speech that cannot be inflected, and a modern meaning, as a function word associated with another word or phrase to impart meaning. Although a particle may have an intrinsic meaning, and indeed may fit into other grammatical categories, the fundamental idea of the particle is to add context to the sentence, expressing a mood or indicating a specific action.
[...]
Particle is a somewhat nebulous term for a variety of small words that do not conveniently fit into other classes of words.

German Wikipedia says

Man rechnet zu den Partikeln – im weiteren Sinne – alle nicht flektierbaren Wörter einer Sprache (wie Adverbien, Interjektionen, Konjunktionen, Präpositionen) oder – im engeren Sinne – nur solche nicht flektierbaren Wörter, die nicht den Unterklassen Präposition, Adverb oder Konjunktion angehören.

That is, the class of particles includes adverbs, interjections, conjunctions and prepositions. It seems that the status of "sogar" is not absolutely clear-cut. Its function is to emphasize something which comes a little unexpected and may be attributed to verbs, nouns and adjectives. Examples:

• Er kann sogar tanzen. [sogar + verb]

• Er kann sogar hervorragend tanzen. [sogar + adjective]

• Sogar er kann tanzen. [sogar + noun]

If it refers to a noun (as in sogar meine Tante), then the word order is determined exclusively by the noun. You can make a test by omitting "sogar" and check whether the remainder is correct. The sentence hatte meine Tante einen Ausdruck is not correct.