If I had been imagining the textbook form of a first sentence, it might have come out like this.
At its core, a story is a character with a problem. Tananarive Due establishes both character and problem in a single sentence here, and adds a location as well.
We have Abbie LaFleur. The name tells us a little already. She's young enough to prefer the -ie ending to the -y ending. That's a mark of informality. LaFleur has me wondering – is she of French descent? Or is the author signaling that she is in someway like or unlike a flower?
Next, we learn her family has been in Boston for three generations, and she is newly arrived in Graceville. Graceville sounds like the southern U.S. to me – possibly due to echoes of Graceland and Louisville – it's a place different from Boston, and clannish enough that the people there see the new arrival as an outsider.
The hyphen in third-generation shows some thought. It's a small detail, and makes the grouping of those words with "Bostonian" easier to follow. As an editor, I appreciated the effort. These bits of craft add up.
Finally, there's the word "warned." It points to a danger that someone should have warned Abbie LaFleur about, and didn't. Since we are working within the genre of horror, failing to warn could have been a sin – the type of moral failing that leads to someone dying gruesomely. Abbie has a problem, and she doesn't know it yet.
In one sentence, the story is wound up and ready to spring.
"The Lake" is one of the stories within The Monster's Corner, edited by Christopher Golden, as well as the first story in Due's own collection, Ghost Summer, and available online at tor.com.