First, syntax is defined in several ways, but a good working definition is this: syntax is (i) the arrangement or order of words, phrases, and clauses in a sentence and (ii) the principles that determine how words, phrases, and clauses can be used (for example, how those elements can be connected). Second, in a discussion of syntax in Hawthorne, it is useful to discuss diction, which is defined as the writer's choice of vocabulary and the arrangement of that vocabulary. In "The Birthmark," for example, diction--Hawthorne's use of 18thC. latinate (Latin-based words)--make the story challenging for modern readers because much of his vocabulary is no longer in use. Syntax itself does not create meaning but is crucial to the creation of meaning when combined with diction (word choice).
Many of Hawthorne's sentences in "The Birthmark" follow the pattern of the story's opening sentence, which demonstrates how Hawthorne moves the reader through the narrative:
In the latter part of the last century there lived a man of science, an eminent proficient in every branch of natural philosophy, who not long before our story opens had made experience of a spiritual affinity more attractive than any chemical one.
The vocabulary (diction), of course, is the real challenge in this sentence, but the syntax is helpful because it is dynamic--that is, (i) the sentence begins with an adverbial phrase that establishes the time period (ii) then establishes the subject ("there lived a man of science"); (iii) then, Hawthorne uses an appositive to further describe the "man of science"; (iv) and finishes with an adjective clause (also, a relative clause because it begins with the relative pronoun who) that tells us what Aylmer did.
In syntactical terms, we call this a cumulative sentence (see entry below for more on the cumulative sentence) because the meaning is not in the main clause--"there lived a man of science"--but in the adjective and relative clauses and appositive that add sufficient detail for us to understand fully what the story will be about--in this case, a dedicated scientist has abandoned science long enough to be attracted to something spiritual, which we soon learn is the beautiful Georgiana.
The crux of the story is Aylmer's attempt to create the "perfect" Georgiana by removing her birthmark, and this theme early in the story in another typical Hawthorne sentence:
His love for his young wife might prove the stronger of the two; but it could only be by intertwining itself with his love of science; and uniting the strength of the latter to his own.
In a variation on the first sentence, Hawthorne begins this sentence with the main clause ("His love. . . ."), but the story's main conflict--the battle between science (Aylmer) and nature (Georgiana and the birthmark)--is introduced in the second clause. The syntax here creates a flow that carries the reader through to the end: the reader cannot stop at the first clause because the syntax propels the reader to the second clause (beginning with the conjunction but), and that is the clause in which the story's conflict resides.
If we look at Hawthorne's syntax as a whole, we will see that, despite the difficult latinate diction, syntax generally creates a forward movement that leads the reader on to the end of a sentence. In many cases, the meaning of a sentence is not presented in the main clause but in all the additions to the main clause. The meaning of the story, then, is not necessarily carried by the main clause but in the adjectival/adverbial clauses, and in Hawthorne's use of appositives.