Refer to the tactics used by soldiers who were armed with bayonets: They dulled the tips of said weapons to prevent them from being lodged in the skeletons of their enemies.
Based on this information, the answer to your question is: Too hard.
As I'm sure you know, you should be trying to bypass the ribcage by slipping the knife between the ribs, with said knife being oriented at an angle which is parallel to the edges of the ribs.
Alternatively, you can slash the neck, the femoral arteries at the groin, and a myriad of other places (which I'm unfamiliar with).
In the way of stabbing you can go down through the soft spot in the left shoulder (next to your neck), up through the jaw (direct path to brain), down through back of neck (to brain-stem), and just about anywhere in the rib cage as it's *almost* all lungs and heart.
Failing those avenues, if the opportunity were to present itself you could sever the nerves running down the underside of an opponent's humorous, thereby crippling that arm's capabilities. You might nix some blood-lines there, too, but I don't actually know.
