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You always subtract the target's AC from the attacker's THAC0 to find the required successful minimum roll. (Or, if you like, add the AC to the roll before comparing to see if you match or exceed the THAC0.) If your THAC0 is 18, you need an 18 or higher "To Hit Armor Class 0." If your enemy's AC is 10, you only need to roll a 8+. If your (very well-protected) enemy's AC is -3, you need to roll (18 - -3) a 21+.
Why can't we just play the game naturally?