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Terran Explained... 
Training
The Single Player Terran campaigns can give you a chance to learn how to use the basic abilities of Terrans. They start out easy, then increase in difficulty while introducing more units and abilities. After mastering those, players wanting to improve their use of Terrans can try the Veteran campaigns. Go to Single Player -> Play Custom -> then select the Campaign folder (first one on the list). Make sure you serve the game type as Use Map Settings. You can choose to start at the beginning and play the whole campaign in order, or head straight to (1)Enslavers3a.scm. This map starts you as Terrans against Protoss and Terrans. This map is very good training for Terran players. While newer Terran players, and even experienced players will find it very difficult, if you continue playing it over and over, you will eventually be able to beat it. As you play it, you learn new and better ways to fight both the Protoss and Terrans. After you are able to beat this mission, try (1)Enslavers3b.scm which puts you as Terran/Protoss against Zerg and Terran. This one is very difficult but it can be beaten.
Construction
A SCV must work on constructing new buildings until the building is finished. During the construction period, the SCV is still vulnerable to attack. If the SCV stops working or is destroyed, the partially completed building will remain until it is destroyed, cancelled, or another SCV starts to work on it again. Only one SCV can build a structure at one time, but multiple SCVs can be used to repair a damaged building.
Support
Terrans require Supply Depots to provide support for their troops. Each Supply Depot provides 8 Supply, and each Command Center provides 10 Supply. It's vital to keep a close eye on your available Supplies to make sure you always have enough to build new troops, especially at the beginning of the game. A halt in the combat unit production caused by not having enough supply available can make a huge difference.
Critical Damage
The hodge-podge circuitry and components used in Terran buildings make them easy to repair but they can burn up if left alone after being severely damaged in an attack. Once a Terran building has been damaged into the "red zone" (reduced to about 1/3 Hit Points, red wire frame) it will slowly lose hit points until it reaches 0 and is destroyed. You must repair it until it is in the "yellow zone" or greater to stop the decaying process of the building. Note that this only applies to Terran buildings, not units.
Upgrades
You can only research one upgrade at a time at any building, and the wide variety of Terran upgrades means that it can take a while to research everything. It's usually a good idea to build at least two Armories (they are very cheap) and you might want to build three to upgrade faster. Multiple Engineering Bays are less useful but could be helpful if you plan on using a lot of Marines.
Lifting Off (or, "That's it, I'm outta here!")
If you look at most the Terran Buildings you will notice that many have the Lift-Off Command available. These buildings are capable of limited flight, and while slow and vulnerable in the air their mobility can be used both for rapid expansion and to flee from enemy ground troops. The Command Center, Barracks, Engineering Bay, Factory, Starport, and Science Facility are all capable of flight.
Buildings may only land on open, buildable land. You cannot land buildings just anywhere. You cannot land it on top of units either, so you cannot crush enemy units with buildings. Buildings have no attack of their own when flying so they are vulnerable to attack, and against most air units they will take even more damage from air-to-air attacks.
Some interesting uses for Lift-Off:
When a resource spot is mined out, rather than just letting your Command Center sit around, you can fly it to another resource spot.
When your buildings are being attacked by melee/ground units only, such as Zealots, Zergling, Ultralisks, Reavers, Tanks, etc. lift-off your buildings. The melee units will not be able to attack them. This can save the buildings until reinforcements arrive, or force the enemy to engage other buildings like Bunkers. This is especially useful with Command Centers as they are very expensive.
In island maps, you can build a Command Center and fly it over to an island without a transport. You can also fly Barracks over early in the game and build a few Marines on different islands to secure them before the enemy is able to transport troops to them.
You can construct a building on lower ground, then fly it up to the safety of high ground to either build units or to keep the buildings safe. Some maps have secondary resource nodes on high ground that aren't accessible from the ground.
If you have a building capable of two add-ons such as the Science Facility and Command Center, and decide you want the other add-on, instead of destroying the old add-on and building the new one, you can lift off and build the other add-on and switch between the two as necessary.
If you find that a Terran enemy has abandoned some valuable add-ons, you can fly your buildings to that location, land by them, and use them. This is of course a rare application.
If you are being nuked, you can lift-off buildings in the targeted area and run before the nuke hits if you are fast enough. You probably won't escape the blast, but you can get away from ground zero and reduce the damage that you take.
You can run away and set up at another location if your town is about to fall. You can return when it's safe, or run away to some other location to rebuild your town. Flee toward allies in allied games.
You can lift-off a building and fly it a bit out of your town to spot for your Siege Tanks, increasing their effective range. If your building comes under attack you can always fly it back into town, repair it, and send it back again.
Remember that you can repair buildings in the air. Placing more SCVs on repairing will repair the buildings faster. If they are being attacked by air only units, you can repair them in the air until they land or get to their destination. This situation comes up occasionally.
Add-ons
There are six add-on buildings for the Terrans:
The Command Center has two add-ons: the ComSat Station for performing sensor sweeps, and the Nuclear Silo for manufacturing nukes.
The Factory has the Machine Shop which allows you to research special abilities for the Vulture and Tank. This add-on is necessary to build Siege Tanks.
The Starport can build a Control Tower add-on which allows it to build Dropships, Battlecruisers, and Science Vessels (with a Science Facility) as well as research technology upgrades for the Wraith.
The Science Facility has two add-ons: the Covert Ops lab for training and upgrading Ghosts, and the Physics Lab which allows you to build Battlecruisers and research the Yamato Cannon.
Add-ons must be used with the building they are designed are for them to work. You can land a Factory next to a Control Tower but it won't connect or function properly.
When constructing buildings capable of having add-ons be sure you keep the right side of the building open when you place it. While it's easy to lift-off the building and move it to a place where the add-on can be built, it's much faster to just build well the first time and leave the spot open so you don't have to move the building.
An add-on belongs to you as long as your building remains connected to it. If you lift-off your building, the add-on building becomes neutral. Any other Terran ally (or enemy) can land a building next to it and attach to it. It is possible to capture or use abandoned add-ons, although this isn't something that happens too often.
If the main building is destroyed, but the add-on remains, reuse the add-on by rebuilding in the same spot. For example if your Factory was destroyed but your Machine Shop was not, select a SCV and instruct it to build a Factory. A special placement box will come up indicating to you where you should build the Factory if you want it to attach to the Machine shop.
Sometimes you might want to use one building and switch between add-on buildings. This can be handy if you're trying to save some money and, for instance, want to alternate between building Ghosts and Battlecruisers. You can move your Science Facility between a Covert Ops lab and a Physics Lab. You can do the same thing with Command Centers if you want to launch a few Nukes, then go back to your ComSat Station.
Building Your Town
With Terrans, it's very important to build a nice tight defense to prevent both attacks on your workers and your buildings. With careful planning, you can build your town such that a few Bunkers can cover the entire area, especially with the U-238 upgrade for your Marines.
If you plan on building Bunkers, build your first Bunker in the middle of the SCVs mining resources. SCVs will be traveling between the Minerals and your Command Center while you're trying to place the Bunker, so it might be easier to stop your workers for a moment while you start on the Bunker. The reason to build it here is this is where the enemy usually tries to attack. If you build your Bunkers elsewhere, the enemy may cruise right past them and attack your SCVs, crippling your economy. If they target your buildings instead of going for the Bunker, you can lift them up and fly them over by the Bunker. If they attack the Bunker directly, you can easily select all the SCVs and instruct them to repair the Bunker as it is being attacked. As the game progresses, add more Bunkers around your town. They should be placed close enough to each other that they set up a nice crossfire. Buildings should be placed around the Bunkers to make them more difficult to reach, especially by melee units like Zealots and Zergling.
Terran
Terran players can Yamato these defenses from a distance using Battlecruisers.
Cloaked Ghosts might be able to sneak some Lockdown casts onto the units up high.
Siege Tanks with Defensive Matrix can be brought in. The Defensive Matrix should give them time to move into Siege Mode. Another Defensive Matrix can be cast on the Siege Tanks if it starts to wear off. A Scanner Sweep can spot for the Siege Tanks so they can fire on the units up on high ground.
Dropships with Defensive Matrix on them might have time to land Marines on the high ground.
Protoss
The Templars might be able to sneak a Psionic Storm on the units above if they are tied up in combat.
A Large group of Carriers or Scouts can break through this defense if the Carriers are not locked down.
Use an Arbiter to cast Stasis Fields on units on the High Ground. Then follow up with a Shuttle landing of Archons or Zealots.
Zerg
Use Guardians to attack these formations from a distances. The Terrans will be forced to come out or die.
Use Queens to Broodling Siege Tanks, Marines, Goliaths, and Vultures.
Use Queens to cast ensnare on the units up there to make them fire slower. This won't touch Marines in Bunkers however (a good spell defense).
Cast Plague on units up there, that should soften the units up enough to attack. A group of SCVs can quickly reverse this spell however if the attack is not immediate.
