This should convince you that it cannot be done without training which at least allows you to mentally internalize useful actions. Everyone should get some training as a matter of course, except that is not so easy either. Rather specific conditions did introduce me to some useful actions which i never have needed. Yet i am the first to note as well that I learned nothing in Basic Training which aimed at conditioning.
You do have to visualize a useful action and even practice it. For a knife attack, you do depend on using the side of your arms to knock the knife aside. That is at least a start. The other clue is to not make the first move but do step back into a defensive crouch if you can. Then drive with the point of your hand for the attacker's throat between the Adams apple and the top of rib cage. This could trigger a reflex action bringing the attackers hands and knife to the throat in protection and even tossing the knife in the process. That needs to be your primary target with bare hands as you will never be in position to do much else. It will also take him by surprise as well.
The bottom line is to have useful actions mentally internalized and try to avoid ever needing them. everything else is generally secondary..
How to Survive a Knife Attack (And 4 Myths That Could Get You Killed)