I'm new and have the AD&D Monster Manual and Fiend Folio. I want to know how to calculate monsters' HP. Does 4 hit dice mean 4 HP, or 4 rolls of a die, and how do I know what die to roll if it is?
Hit dice in AD&D are d8 unless otherwise specified. A creature with 4 hit dice would have 4d8 hit points. That means that you roll a d8 four times and add the results together to generate its hit points.
HIT DICE indicates the parameters of the number of hit points a creature can withstand before being killed. Unless stated otherwise hit dice are 8-sided (1-8 hit points). The indicated number of dice are rolled, and the numbers shown on each are added together to arrive at a total number of hit points. (MM1e p.5)