This is easy.
One way is to cut the onions such that the side that you are cutting faces away from you. However it doesn’t always work.
To prevent tearing, you can cut the onions in halves first and than soak them in iced water for 5 minutes. Only dice them after that.
Another method is to store them in fridge. Onions release a volatile vapour when they are cut, and that’s what caused the tears. Keeping the onions cold retards the release of the vapourиконииконописikoniсвети георги