英文倒裝句的結構 句型分析?

A dog is under the table.= 主詞＋動詞＋主詞補語（simple sentence)
Under the table is a dog.= 主詞補語＋動詞＋主詞 （倒裝句構:主詞還是dog，只是倒裝了）
There is a dog under the table.有隻狗在桌子底下。（這樣寫，句子較自然，因為句首不直接出現新資訊，而用副詞there開始，there不是主詞，而是主詞補語）

虛位主詞（anticipatory）僅有文法功能，是「形式上」的主詞，沒有實際意義，如下列各句的it：
It is nice to be home again.不定詞to be home again是delayed subject（延遲的主詞）
It has puzzled the public why the government can't solve the traffic problems.(名詞子句why the government can't solve the traffic problems.是延遲的主詞。
延遲的主詞是「實質上」的主詞。採用虛主詞的方法來寫，可避免主詞較長，讓句子不會頭重腳輕。
這種採虛主詞的句子在做文法分析時，仍視為原來的句型（即同無虛主詞時的句型）。

「受詞」是不定詞片語或名詞子句時，也有向後移動到「受詞補語」之後的現象，以it做為「虛受詞」：
We thought it our duty to take care of the orphans.（不定詞片語to take care of the orphans是延遲的受詞）
I find it amazing that anyone should want to listen to such rubbish. （名詞子句that anyone should want to listen to such rubbish.是延遲的受詞）
這也是英文會將較長的文字向後移位的習慣現象。

"Inversion" is altering the natural order of words from famous English persons, writers;eg:-
Under (it) A dog is.
Under (it) is the imaginary "it" as subject, instead of main subject---(A dog) which does not change, before and after the Inversion style of figures of speech from your Five Great Pattern Analysis (?)
----Hence (it)=the table;
----Hence A dog=the object;not the subject as you interpreted at first !
The natural order is "A dog is under the table", ie. (s v o ) styled from the figures of speech seen from the famous English persons, writers.