Umm, screen tearing happens when the frame rate and the monitor refresh rate don't match. When that happens, you can, sometimes get issues where you see half of one frame in the top half of the screen, and half of the previous frame in the bottom half. If that's happening, you turn on v-synch and the problem goes away (frame rate gets rounded down to a multiple/factor of the refresh rate, which massacres your frame rate when you have a 60 Hz monitor and your frame rate drops to 59.9 fps (down to 30, then 20, 15, 12, 10...). So don't use v-synch unless you're having the tearing issue. THIS, on the other hand, is a much bigger problem. You seem to have numerous opinions in other comments suggesting the same things I would.
It's hard to explain all the variables that can cause screen tearing. Your monitor is part of it. Its refresh rate matters. The game matters a lot. Imagine that the top half of your screen is refreshing perfectly fine. But when you turn around all of the objects (ex: a busy city) give the computer a bit of an Oh Nelly! and suddenly the top part of your screen slows down a bit. Meanwhile the bottom half refrehsed just fine because there are few objects to see (ex: a road texture). So the screen seems to cut where the slowed down half and the usual speed half meet.
显示效果：https://www.youtube.com/watch?v=jVAFuUAKPMc