Huh? Isn't falling, by definition, something you can't predict?
Yes. But you can develop your ability to fall better.
Falling while trail running is an inevitability. Most people just do their best to avoid falling and hope they don't get hurt too badly if the unfortunate happens. I'm clumsy. I don't like to take those chances.
So how do you go about learning to fall? Find a location with soft ground. Sand is perfect. Grassy fields are another good choice. Run at a slow speed, then purposely fall on the ground. I prefer to use a “slow your fall with your arms, then roll” technique. As you're falling, keep your elbows bent. When you hit the ground with your hands, the bent elbows will act as shock absorbers. As my arms are absorbing the shock, I begin rolling my body to the side away from the most dangerous debris. Depending on the trail, I may roll several times.
This specific technique will not work on all trails. For example, it may be impossible to avoid serious injury if falling on rocky mountain trails. In that case, do what you can to avoid smacking your head on a sharp rock.
I prefer to carry handheld water bottles to help soften the blow of landing hard on my hands. The bottles usually take a beating, but it saves my hands. Wearing gloves can also serve the same purpose, but may be too hot depending on the weather.