There are 2 moments in the game in which you can perform a fatality - after defeating Scorpion and Sareena. But you must know that they're crucial to further events. So not spoiling anything, I'll just suggest you try completing the game with and without them to see the differences in plot development. Also, you may want to try various difficulties. Believe me, it's worth it
The combination for Sub's fatality is
F-D-F-HP one step from kneeling opponent.
These are additional passwords you may find interesting:
- GTTBHR - Infinite Lives
- NXCVSZ - 10 urns in your inventory
- RCKMND - Hidden movie
- CRVDTS - Credits
- ZCHRRY - Then hold L1 during loading time to fight Quan Chi and L2 for Shinnok.
As for combo question, the 6 hits one (HP-HP-LP-LK-HK-B+HK) is the biggest ground combo. However, there're special conditions in which you can perform more damaging ones, but they aren't too useful. So keep using that trademark Sub-Zero's combo until you get the Ice Shatter move.
BTW, what is your biggest combo in this game guys?