You must use the keyboard during combat to carry out most combat commands.  For more information on which keys to use, see the Commands section of this On-Line Information System.  You can also set the default fighting option for each character in the party with the Party Options window (see Parties for more information). 

If auto-fight isn't turned on (default) in the Party Options (i.e. begin fighting as soon as non-peaced monsters are encountered), the player can 'turn on' fighting by pressing 'f'.  Keep in mind that if the monsters are at war with you, they don't care what you do - they'll attack you every round.  Sometimes a character may want to stop fighting to allow other party members to kill the monsters and gain experience - one can turn off fighting by pressing 'd', which will cause the character to Defend only. 

Defend is an option usually only used when in a Party, however, if a player is experienced enough, they can use Defend when running through a room right before a monster strikes.  Defend will increase a character's Defense (while selected) by 50% during combat. 

Fleeing Monsters 
Some monsters may try to run during combat.  This usually happens if they feel outnumbered, out powered, or are downright afraid of you and your companions.  Wounded monsters will also be more apt to run.  The chance of a monster successfully running away depends on how fast the members of your party are. 

As you adventure, you will notice that some creatures tend to be more frightened while others will never run away.  Monsters who have chests are usually less apt to run away and will usually defend their treasure with their life. 

Pausing Combat 
Trying to regulate a party of four characters during combat with 2 seconds to assign orders is sometimes just too much.  For this reason, the Pause feature was added, which allows the player to halt combat and assign actions for the next combat round.  The player can activate Combat Pause by pressing shift-p during combat.  Once orders (i.e. cast spell, fight, defend) have been given, the player can press the displayed OK button.  Note that all movement, etc. is restricted when in combat pause mode -- so if you're going to run, you'll need to do it in real time! 

You cannot pause the game in Multiplayer mode, nor should you need to, as you only control one character.

Additional Information 
It is in combat that your character gains the most experience.  The tougher the monster, the more experience your character will gain. 

Sometimes, your character may become "pinned”.  This means that your character cannot learn any more (gain experience) until they reach the next level in their current guild.  It also means that your character has enough experience to make level and is 1 experience point away from making the next level.  This quite often happens when a character is looking for a quested monster.  The only way to become "un-pinned” is to make a level.