In this tutorial I will, assuming you have prior knowledge of DEdit, explain some of the simplicities of making triggers.
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Triggers are a core piece for any singleplayer map, they allow you to modify anything from a sound playing to an alien ripping a vent open and trying to eat you up to the global fog of your map.
These are some examples:
This message goes to an AI character.
TargetName1 : AI_Bob
MessageName1 : scr (igd;rot player;say 29533;end;);end
This message goes to the player.
TargetName1 : Player
MessageName1 : fadeout 2.5;delay 3.0 (fadein 2.5;);
What kind of input does an object need exactly? - by this I mean does the door object need the input TRIGGER or OPEN to open it up?
Well, this list should clear up some confusion.
Doors require TRIGGER to be opened and closed - You can also use TRIGGERCLOSE to have the door only trigger when the door is in the OPEN state, useful for ensuring a door is shut when required.
Triggers can use TRIGGER to be set off by other objects, useful if you require one object to cause multiple events, in which you may require the trigger object to handle more commands.
Explosions require TRIGGER to be set off.
Spawners (AI, Debris etc) require ON to start spawning.
Cameras require ON to start viewing.
Sounds require ON to begin playing.
For example, a 'trigger' message would be:
TargetName1 : Door0
MessageName1 : TRIGGER
A 'ON' message would be:
TargetName1 : SoundFX0
MessageName1 : ON
Hopefully this should clear some things up and give you a better idea of how it all works, using triggers you can create some pretty complex stuff, from (as put earlier) an alien ripping open a vent and trying to eat you, to some huge cinematic cutscene for your lengthy campaign based on some distant, bug-riddled rock.
Any questions? Post here and I'll reply.
-Rasirez