I am an absolute beginner, trying to learn.
Let's consider that board (from a tutorial at http://playgo.to/iwtg/en/):
It's black's turn to play. The topic is to avoid a false eye at D9.
According to the tutorial, if I play at E8, I do not have a false eye:
But, if I play anywhere else and white plays at E8, I do have a false eye:
I do not understand how having a white stone, at the diagonal, would make a false eye. Can anyone explain?