I'm not familiar with SC2's map editor (I was pretty good with BW's), as I can't even figure out how to do many of the simplest tasks.

For instance, I want the trigger to create a tank every second, only if there is a factory present. I know how to make units periodically, but I don't know how to make it so that the factory has to be present. I think it should be an event, but I can't find an event that is to the effect of "player owns unit". Any help?

I appreciate it.

You can do that in the Data Editor, which might be easier. Basically you could set up the Factory so that it creates a Tank every second. Or did you need the Tank created at a different location entirely?

No, that should be a good solution. Now hopefully I can figure out the data editor...

EDIT: Nope. I miss BW

Would anyone be able to tell me how to make a factory autotrain a tank every second?

sigh... im so happy ppl atleast wait til going over to sc2mapster before asking these questions.
Ok so basicly i would like to say that u have to sstart WAAAAY more at the begining since this is a level 2 trigger (meaning it includes variables). It is better to do all things though data editor ( or atleast cleaner) but more LOGIC to do it in a trigger. And since its easier to understand i will explain triggerwise. before we start i would like you to not only do, but to try and understand.

Basicly, if i understand correctly u want to spawn a unit every sec IF (a condition) you have a factory (or a structure of some sort)

Since i dont have time with a fuckin guide ill just print out what i would do in 3 easy tiggers. First one (Player makes a factory) Second one (Spawn units) third (Factory dies)

Now first u need 2 varaibles:

Step 1: variables
First one- Call it.. FactoryExcist - (make it global (meaning your NOT making inside the trigger, but in the left spalt (where all your triggers and varaibles are)) should be a boolean type. Its auto made to false (which is ok). Under the type (as u just set to boolean) there is something called "array". check the box and set the array = numbers of players (if 2 players, then set it to 2).
This trigger is very usefull. It can be set as false and true and used in triggers to open let a specific player pass a condition, while another doesnt. (false, true.. very logic)
The SECOND - call it Factory unit - trigger should be a global one aswell (opposite would be if u made it WITHIN the trigger making it local - but dont do this). Make this global trigger a unit type of variable and set the array to number of players.
This trigger can store a specific unit (structures are counted as units, and be used to pin out what unit we refear to when telling the map where to.. for example spawn certain units).

Step 2: Building a factory

Events
Unit - Any Unit construction progress is Completed
Local Variables
Conditions
(Unit type of (Triggering unit)) == Factory
Actions
Variable - Set FactoryUnit[(Owner of (Triggering unit))] = (Triggering unit)
Variable - Set FactoryExcist[(Owner of (Triggering unit))] = true

Step 3: Spawning units (this trigger has to be under the first one in the list to the left)

Events
Unit - Any Unit construction progress is Completed
Local Variables
Conditions
(Unit type of (Triggering unit)) == Factory
Actions
General - Wait 2.0 Game Time seconds
General - While (Conditions) are true, do (Actions)
Conditions
FactoryExcist[(Owner of (Triggering unit))] == true
FactoryUnit[(Owner of (Triggering unit))] != No Unit
Actions
General - Wait 1.0 Game Time seconds
Unit - Create 1 Siege Tank (Tank Mode) for player (Owner of (Triggering unit)) at (Position of FactoryUnit[(Owner of (Triggering unit))]) facing 270.0 degrees (No Options)

Step 4: Factory dies..
Unit dies
Events
Unit - Any Unit dies
Local Variables
Conditions
(Unit type of (Triggering unit)) == Factory
Actions
Variable - Set FactoryExcist[(Owner of (Triggering unit))] = false
Variable - Set FactoryUnit[(Owner of (Triggering unit))] = No Unit

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There.. Havent tried it but should work.. Now If the dude need help how to do the action and functions, please just help him..
As i said: This is a trigger way to do it, and often more simple to understand then data editor. But for next time, try reading guides..

@post above this
-You do things in the trigger editor because you can easier control/tweak them; but triggers cause more traffic than data editor's aspects I think (only noticeable, if you have bigger amounts of data changes)
-Your triggers will create a thread for every unit (which is a bad idea because the system is totally unefficient and you will hit the thread limit). Having one trigger that periodically creates the units would be the best trigger solution.
-You trigger can only support 1 factory because you store the units in 1 entry (you will overwrite factories before -> use dynamic storage system).
-You can do that in 1 trigger without any global variables in your design (without the thinking mistake described 1 lime above this line).

@topic
If I would trigger that, I would use 1 unit group for the factorys.
-1 trigger like his first one to add the building to it, if the constructed building is a factory.
-1 trigger that deletes dieing factorys out of the unit group
-1 trigger that periodically creates the spawned unit for every unit in the unit group at its position and its owner.