Hello, Katherine:
I love to diagram sentences. May I try?
As you said, the main sentence is:
The = adjective (or determiner in newer grammars)
windmill = noun (as subject)
was used = verb (passive)
on the prairie = prepositional phrase modifying the verb
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As = I think that you could find a million definitions for "as" here. Some would call it a preposition; others would label it an expletive. One thing for sure: it is not a conjunction, for -- as you said -- there is no verb.
the = adjective.
invention = noun.
that brought deep well water to the surface = As you said, those words are an adjective/ relative clause modifying the noun "invention." (It answers the question: what invention?)
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Now for the $64, 000 question (that was a lot of money when I was young!): What is the role of "As the invention ... water to the surface"?
1. Your book says that it is a dependent adverb clause. I do not understand what that means, and -- of course -- I
would never dare disagree with what a book says.
2. I think that Tdol put his finger on it when he suggested that the whole clause refers to the windmill.
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Now here is exciting news: Let's temporarily erase "that brought well water to the surface."
We get:
As an invention, the windmill was used on the prairie.
I think (repeat: think) that most books would say "As an invention" is an appositive that modifies the whole sentence "the windmill was used on the prairie.")
Here is a similar sentence from the great grammarian George Oliver Curme:
As a first step, I secured my vast property, so that the income would be certain.
Professor Curme explains that such an explantory remark belongs to (modifies) the whole sentence.
Now I suggest that maybe we can say "As an invention (that brought well water to the surface)" is an appositive
(explanatory remark) that belongs to the whole sentence "The windmill was used on the prairie."
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If we erase the "as," "everyone" would agree that it is an appositive in:
The windmill, the invention that brought deep well water to the surface, was used on the prairie.
But for the sake of style, perhaps it is more elegant to put it at the beginnning of the sentence and introduce it with
"as."
P.S. Maybe your book uses the word "adverb" because when we add the word "as," the clause takes on an adverbial sense in that it tells us why the windmill was used on the prairie.
