Woolf's To The Lighthouse is modernist because it experiments with the novel form. First, Woolf tells the story through stream-of-consciousness narration. This means there is no omniscient, objective narrative voice telling us what is going on: the story is told through the subjective thoughts of various characters. This creates the fragmented feeling of the novel and captures the way most of us experience reality: through the unreliable filter of our emotions and desires.
The novel also experiments with time. The bulk of the novel covers one afternoon and evening at the Ramsey's beach home on the Isle of Skye as the parents discuss whether they can take a boat out to the lighthouse the next day, Lily works on a painting, and Mrs. Ramsey carries off the triumph of a successful dinner party. Then ten years pass in a short linking section, and in the final section, the trip to the lighthouse, much belated, takes place. The novel ends, however, as the party arrives at the lighthouse, symbolizing that the journey is more important than the destination.
This experimentation with time allows Woolf to avoid Victorian and Edwardian novelistic conventions, especially the kind of deathbed scene that played such a prominent role in Victorian fiction: Woolf was determined that Mrs. Ramsay's death would take place entirely off stage and not be sentimentalized.
The combination of stream-of-consciousness narration and the stretching and condensing of time startles us and jars us out of complacency. By destabilizing conventional notions of realistic fiction, it reflects the questioning of authority that took place in the aftermath of the disaster of World War I.