A Primer to 2nd edition combat

 The Combat Round
1) DM decides what the monsters will do

2) Players state what they will do

3) Initiative is rolled.

Use 1d10 - low is good (fast)
Add weapon speed or spell casting time
Add other modifiers:

Reaction adjustment (DEX)
Magic weapons (+1 weapon gives -1 initiative)

Hasted -2
On higher ground -1
Set to receive charge -2Slowed +2
Slippery footing +2
In deep water +4
Submerged +6
Hindered +3 (entangled, climbing, held)

Natural weapons (small and medium) +3
Innate spell +3
Misc magic +3
Potion +4
Ring +3
Rod +1
Staff +2
Wand +3
Scroll (spell casting time)
Waiting +1

4)Actions are resolved in order of initiative

Note:
Offhand attacks are made at the same time as the main hand attack.
Additional attacks due to level or specialization come when everyone else has had one attack turn.
Spells with casting time of 1 round take effect at the end of the round, when all over attacks and actions are done.

5) New round, start over at 1)

What can you do in a round?
Full attack (and move 1/2 movement rounded down)
Cast a spell (and move 1/2 movement rounded down)
Move full movement
Drink a potion (and move 1/2 movement rounded down)
Light a torch (and move 1/2 movement rounded down)
Use a magical item (and move 1/2 movement rounded down)
Bind a character's wounds (and move 1/2 movement rounded down)
Search a body (and move 1/2 movement rounded down)
Recover a dropped weapon (and move 1/2 movement rounded down)
Make a Missile attack (full rate of fire)
Make a missile attack (half rate of fire) (and move 1/2 movement rounded down)
Charge someone
Withdraw (move 1/3 movement rate) away from enemy, provoking no AO's
Flee (double move, provoking AO's)
Stand up and attack (or move half movement rate)

Stuff like switching weapons, shouting orders, dropping stuff does not cost anything.

Surprise
Roll 1d10 - low is bad. 1-2-3 = you are surprised.
Modify for DEX (reaction adjustement)

If you attack someone surprised, you gain +1 to hit
If you are surprised, you gain -2 to saves
If you are surprised you gain no DEX bonus to AC

Neither attackers nor defenders can use spells during a surprise round, but magic items is okay


THAC0
Roll 1d20 - high is good.

Modifiers +'s are good, but you subtract them from your base THAC0
Fx: Base THAC0 14 and +3 from STR 18.00 and +2 from magic weapon = THAC0 9, meaning +11 to hit.

For me, the easiest way to calculate which AC I hit is as such:

Roll a 15.
15+11 = 26
20-26 = -6
You hit AC -6

or

Roll a 4
4+11 = 15
20-15=5
You hit AC 5

You can also do the following:

Subtract the die roll from your THAC0 or THAC0 from your die roll - just make sure the biggest number is first. If roll is bigger THAC0 number, then you hit a negative AC. If THAC0 number is bigger than roll, then you hit a positive AC.

Roll a 15
15-9 = 6
Since 15 > 9, you hit AC -6

Roll a 4
9-4 = 5
Since 4 < 9, you hit AC 5


Misc. attack modifiers
Higher ground +1
Defender invisible -4
Defender off-balance +2
Defender sleeping or held Auto-crit (HOUSERULED)
Defender stunned or prone +4
Defender surprised +1
Missile fire long range -5
Missile fire short range -2
Rear attack +2

Cover
25% hidden -2 to hit
50% hidden -4 to hit
75% hidden -7 to hit
90% hidden -10 to hit

Concealment
25% hidden -1 to hit
50% hidden -2 to hit
75% hidden -3 to hit
90% hidden -4 to hit

Spellcasting
No DEX bonus to AC while casting spells
HOUSERULED: No need for to hit against willing target.
If hit by attack (attack does not have to deal damage) before spell is complete or if fail a save before spell is complete, the spell fizzles.

Saving throw
Roll 1d20, high is good.
High DEX gives bonus to BW and saves to avoid stuff, such as fireballs etc.
High WIS gives bonus against illusions, charms and other mental attacks.
Magical rings and cloaks of protection give bonuses against all saves
Magical armor gives bonuses against any kind physical effect.

Hope it helps remembering ;)

Edited the mistake regarding saves