A lot of arts make it look a lot easier than it really is but it is also not as impossible as others in this thread seems to believe.
The main trick is accepting the fact that you are going to get hurt. Get past that then you can focus on minimizing that hurt by taking the other guy out as fast as possible.
[aside]
It can be a lot of fun to play with this with a buddy. Go to staples and get a 12-pack of coloured sharpies. Your buddy gets a sharpie and you get nothing. Spar with your buddy with him/her seriously trying to mark you (none of that "telegraphed motion" crap most people use in regular practice). You win the round if you make your buddy drop the "knife" or he ends up on the ground with you still standing. Your buddy wins if she marks you in any one of [your core, neck, back of your knee, hamstring, inside of arm] or marks you more than twice anyplace else. Use a different colour each round so it's easier to keep track.
After you use all the colours, switch roles.
[/aside]
Note that the OP had a highly trained unarmed vs an incompetent knife. That's way easier.
Most people aren't saying it's impossible. I'm not.
In practice, I can actually do fairly well against unskilled people. I can disarm/stab them maybe 50-60% of the time, unarmed vs knife. The highly skilled people can win 90-95% of the time against newbies, 80% of the time against me.
The main trick is accepting the fact that you are going to get hurt. Get past that then you can focus on minimizing that hurt by taking the other guy out as fast as possible.
[aside] It can be a lot of fun to play with this with a buddy. Go to staples and get a 12-pack of coloured sharpies. Your buddy gets a sharpie and you get nothing. Spar with your buddy with him/her seriously trying to mark you (none of that "telegraphed motion" crap most people use in regular practice). You win the round if you make your buddy drop the "knife" or he ends up on the ground with you still standing. Your buddy wins if she marks you in any one of [your core, neck, back of your knee, hamstring, inside of arm] or marks you more than twice anyplace else. Use a different colour each round so it's easier to keep track.
After you use all the colours, switch roles. [/aside]
Note that the OP had a highly trained unarmed vs an incompetent knife. That's way easier.