It is do or die when it comes to opening sentences in flash fiction, and it all happens in the first line you give the reader. Show the conflict right there, and remember as a flash fiction writer your job is to tell a story from the very beginning. Find a great line and you can carry the reader all the way through your story.
I found these great first lines on this website: American Book Review:
‘Call me Ishmael.’ —Herman Melville, Moby-Dick (1851)
‘Lolita, light of my life, fire of my loins.’ —Vladimir Nabokov, Lolita (1955)
‘It was a bright cold day in April, and the clocks were striking thirteen.’ —George Orwell, 1984 (1949)
‘The sky above the port was the color of television, tuned to a dead channel.’ —William Gibson, Neuromancer (1984)
‘It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife.’ —Jane Austen, Pride and Prejudice (1813)
As an author(s) many of us could launch into a story with any of these opening lines. They’re good. They’re great lines, and there is a reason why they have stuck around all of these years. These lines launch the reader immediately into the story. As writers, we need to do this also.
If you need more examples of great opening lines you could google ‘Great First Lines in Literature.’
Or go to this site: