"Hook the reader" is the most common advice given about opening lines, and it's almost useless on its own, because it describes an effect without explaining a method. A first line has three actual jobs, and a sentence that does even two of them well will outperform a sentence that's merely dramatic.
Before a reader cares what's happening, they're unconsciously deciding whether they want to spend the next three hundred pages with the person telling them this. Word choice, sentence rhythm, and the narrator's attitude toward the material all do this work before the plot does anything at all. A flat, competent sentence about an explosion is less compelling than a strange, specific sentence about nothing happening yet — voice beats event.
Every effective opening line leaves something unresolved — not necessarily suspenseful, just incomplete. A name without context. A claim that seems exaggerated. A relationship described before it's explained. The question doesn't need to be big; it needs to be specific enough that the reader notices it's missing an answer.
Abstraction is the enemy of a first line. "Everything changed that year" promises nothing a reader can picture. A first line that names a real object, a real place, or a real, specific action gives the reader something to stand on while the voice and the question do their work around it.
A strong opening sentence is often short, oddly specific, and quietly confident — it doesn't explain itself, and it doesn't try to be the most dramatic sentence in the book. It just has to make the second sentence necessary.
If you're stuck on a first line, try writing the most boring, literal version of your opening scene, then ask which single detail in it the reader hasn't earned an explanation for yet. That unexplained detail is usually your real first line. IARobo's Story Continuation tool can also take a rough opening and suggest five different voice-driven variations to compare side by side.
Want to put this into practice? Open IARobo and try it in the Novel Generator, Poetry Studio or Character Creator.