fail
To not succeed at something you tried to do.
When you fail, you try to do something but don't succeed. A baker fails to make bread rise if she forgets the yeast. A basketball player fails to make the shot when the ball bounces off the rim. A student fails a spelling test by getting too many words wrong.
Failure happens to everyone, and experiencing it doesn't make you a failure as a person. Thomas Edison failed many times while inventing the light bulb, but each failure taught him something new about what wouldn't work. Scientists fail in a lot of their experiments, which is how they discover what's actually true. Athletes fail to win many of their competitions, but the practice makes them stronger.
The word can also describe things breaking down or stopping. When car brakes fail, they stop working. When someone's courage fails them, they suddenly feel too scared to act.
Some people avoid trying new things because they're afraid to fail. Failure is often how people learn the most. A mathematics student who fails to solve a hard problem on the first try learns more by studying her mistakes than she would by only doing easy problems she already knows how to solve.