>>115721939You will note that it's AC is 8. Your 1st level character has a THAC0 of 20. In order to calculate the target number you need to roll on a d20 to hit the beast, you will need to subtract the Gelatinous Cube's AC from your THAC0 (20 minus 8). 20 - 8 = 12. So you now know that you need to roll a 12 or higher on the d20 to successfully make a hit.
The weapon you're using may have a modifier (most weapons have a modifier). Lets assume that the sword you're using is well crafted, and has a +3 modifier (this attack-roll-modifying-number helps you to hit targets). So subtract the swords +3 modifier from your THAC0 of 17; which is now a 14 (15 THAC0 vs. -2 AC = 17 THAC0 - +3 modifier = Target Number 14), so you need to roll a 14 or higher on a d20 to hit the beast. Note: there are also other types of modifiers that can come into play.
For example, lets assume your character (THAC0 15) is attacking a -2 AC monster, and with the +3 sword you're using your THAC0 is a 12. Since this monsters AC is -2, you add it to your THAC0; you have now calculated that the target number is 14 (THAC0 15 - +3 sword modifier = THAC0 12 vs. AC -2 = target number of 14).
Remember to add a targets negative AC number to your THAC0 as opposed to subtracting a targets positive AC number from your THAC0 when calculating the target number you need to equal or beat in order to hit.
When you hit, you then roll for damage according to the weapon you're using (for example, if you attacked with a dagger, you would roll a d4 to determine how much damage you deal). Each weapon or spell that you can attack with will list the type of dice you use when rolling for damage.
