Can V-Sync avoid tearing? I think it can't. Am I wrong? 1) Update the front buffer will cause screen tearing, every one knows it.

2) But V-Sync, I think it still get the same issue. Why I get this idea? 

2.1) When GPU flips to the back buffer, current screen displays the old image which was in previous front buffer.

2.2) Now GPU sends image from "current front buffer" (previous back buffer) to monitor. This will last 16.7ms (60Hz). During this period, Human eyes will see half "current front buffer" and half "previous front buffer". Is it right?

2.3) If it is right, it would be same as non-V-sync.

3) But, in fact, when I switch "V-sync" on, I can't see screen tearing. Why?

Thanks a lot for the answers.
V-sync means that the GPU won't switch buffer until the current frame is done showing. 

When you tell to swap the buffer with V-Sync enabled the actual swap doesn't happen until V-Blank, a small window between frames where nothing is being sent to the screen.

The reason I think you cannot see the update tear is that it moves across the screen during the update. While a non v-sync tear stays in the same place on the screen for 16 milliseconds.