What is syntax? It's simply the grammatical structure and the pattern of words in a sentence.
What's fascinating about language (at least for geek writers like me) is how syntax is your plaything. You can try practically any arrangement of words. Where should I put the part of this sentence that I want to emphasize the most? At the beginning? The end? Why? Which sounds better? Which sounds more memorable?
Does a sentence sound better when broken up into two or three separate parts, or does it function better as a single, albeit more complex, sentence? Hint: the shorter and simpler the better. It's more forceful and easier to understand.
Is there a rhythm to the sentence that is amplified (or ruined) by a certain word choice or word order? And does that rhythm distract the reader--or help her understand?
Of course in poetry, there are even fewer rules of syntax. Which is both a good and a bad thing:
My mistress' eyes are nothing like the sun
This line, from Shakespeare's Sonnet 130, is wonderfully rendered from a rhythmic standpoint, but the meaning isn't clear at all. What does it mean to be "nothing like the sun?" Does that mean dark? Not yellow? What is his point?
Of course that's the key problem of rigid poetry structures like the traditional sonnet. They force even the greatest writers into bizarre syntax, and it only serves to confuse the reader.
Which leads us to an important truth about writing: Never put form before function. The great blessing of prose writing is that there are no structural requirements whatsoever. There are only some loose and usually breakable rules of grammar that we can manipulate at our whim. Which allows us to craft sentences that say, clearly and with force, exactly what we want them to say.