- There is a book on the table -- existential clause
- A book is on the table -- basic version
- A book is there -- (there= on the table, adverb of place) An existential clause is a clause that refers to the existence or presence of something. If something exists in some place then we use adverbs of place. So adverbs can fulfill the use of existence or presence of something. Then why we need dummy subjects and its usage?
- A book is there ~ There is a book. If we point out that thing (here "the book") and tell that " There is a book", here "there" is not used as dummy subject but it is an adverb. The normal word order in positive sentence is “A book is there” (subject-verb-other components) but here “adverb-verb-subject”. Can we use this order? And which order is the best to use and why?
I'm not sure I understand. I don't think it's necessarily a dummy subject; I think it's an inversion or, at least, in its original sense, it was probably an inversion:
"There is the book!" (at that place is the book)
"Here is the book!" (at this place is the book)
"There are three people here." (This is perhaps a dummy subject)
I'm assuming the dummy subject that you're talking about is just an idiomatic expression that's been around so long in English that no one questions it; it's probably from a time in English when "there" could be used to mean "here" as well. In fact, in spoken French, "là" means "there" and "ici" mean "here"; however, one usually uses "là" for both "there" and "here" and "ici" is reserved more so to point to one thing that is closer if one should be discussing two different things. I'm sure it runs along that kind of logic. The French idiom for "there is/are" is "Il y a", which literally means "it there has" or "it has there" (yes "y" can mean "there" as well).
Some idioms just exist and that's the reason. There was a lot of inversion in Old and Middle English, so that might be the reason that this "idiom" exists. Have you ever wondered why it is technically "It is I" instead of "It's me"? Well, in Middle English, the inversion was "It am I" and it slowly started being misconstrued as people thought the subject was "It" when really it was an inversion and the subject was "I". After this, many people didn't like seeing a nominative pronoun in the predicate when, normally in English, there is an accusative pronoun, so this disjunctive formation using "me" came about; thus, we have "It's me" as the colloquial form of the very formal "It is I".
I hope that might have helped you out. Take care and good luck.