O. Henry starts his story off with a sentence fragment:
ONE dollar and eighty-seven cents.
He then follows with two very short sentences and then a long sentence:
That was all. And sixty cents of it was in pennies. Pennies saved one and two at a time by bulldozing the grocer and the vegetable man and the butcher until one’s cheeks burned with the silent imputation of parsimony that such close dealing implied.
Throughout the story, O. Henry moves back and forth between short, often abrupt sentences and longer, more meandering sentences. This variation in sentence length helps maintain the reader's interest and moves the story along.
The diction when Della's immediate thoughts are captured is simple, as it follows the flow of this young and good hearted woman's mind as she copes with her strong feelings of disappointment that she doesn't have more money to buy Jim a Christmas present. The long sentence reflects her build-up of emotion at how hard she has tried to save and then trails into a more complex diction—"the silent imputation of parsimony that such close dealing implied"—that reflects a slide into the narrator's summation of her thoughts. (This is not diction we imagine Della using.) But then again, we slide back into the simple diction or speech pattern of Della's immediate thoughts. This helps us identify with her:
Three times Della counted it. One dollar and eighty-seven cents. And the next day would be Christmas.
The story is organized primarily around Della's point of view. It is as if a movie camera, through most of the story, is following her. We see her go and sell her hair. We watch her watching Jim's reaction to her hair cut as if we are seeing him through her eyes. Only at the end, in the very last paragraph, do we move from Della's point of view to the final words of summation of the moral of the story by the narrator.
The tone of the story is both sweet and poignant as well as ironic. Despite the irony, this is a heartwarming tale.