"not only" na początku zdania z lub bez inwersji

W mojej książce jest taki oto fragment:


Nie łapię tego. Jeśli po początkowym "not only" należy postawić operator to dlaczego "not only the bathroom was ..." a nie "not only was the bathroom ..."?

bo bathroom jest podmiotem, a w drugiej czesci zdania wlasnie podmiot jest wymieniony na inny
Porownaj
Not only was the bathroom flooded, but it was also dirty.

Czy chodzi o to, że inwersja w pierwszej części jest obowiązkowa tylko kiedy zarówno po "not only", jak i po "but also" jest pełne zdanie?

Czy może być "not only was the bathroom flooded, but also the rest of the house." ?

'pelne zdanie' to tylko konsekwencja tego, co pisalem o podmiocie, zreszta zamiast '...rest of the house...' mozna napisac 'but water also stood in the other rooms' i masz pelne zdanie bez inwersji w pierwszej czesci

Zrozum to, co napisalem o podmiocie.

Staram się. Czy możesz odpowiedzieć tak/nie na dwa poprzednie pytania?

odpowiedzialem na pierwsze. odpowiedź na drugie będzie jasna, gdy zrozumiesz to, co pisałem o podmiocie.

(1) Not only the bathroom was flooded, but also the rest of the house (was flooded).

In the first clause, “only the bathroom” is within the scope of “not”; in other words, “not” negates “only the bathroom.” “Not” does not negate the flooding of the bathroom.
Try to find this reading one step at a time: The bathroom was flooded. Only-the-bathroom was flooded. Not-only-the-bathroom was flooded.

The conclusion you should draw at this stage: in (1), the combo “Not only” does its thing to the subject “the bathroom” only. It naturally fronts the subject. “Not only”(with auxiliary) and “the bedroom” do not trade places; hence there is no negative subject-auxiliary inversion (NSAI) here, and the word order in the clause is canonical.

(2) Not only do they need clothing, but they are also short of water.

Find the reading of the first clause in (2):They need-clothing. They need-clothing-only (or They only-need-clothing). They not-only-need-clothing (because they need more than clothing). Here “not” negates “only-need-clothing”
For emphasis, we pull “not-only” from behind the subject “They” to the front of the sentence. But “not only” won’t go alone and drags the auxiliary with it to complete NSAI.

Czy może być "not only was the bathroom flooded, but also the rest of the house." ?

No.
With no inversion: The bathroom was not only flooded, but also the rest of the house???