General PvT Strategy
From Liquipedia Starcraft 2 Wiki
 Early Game
In the early game of PvT the Protoss generally is fighting to survive long enough to safely expand and get tech to defend later timing attacks. Early attacks with Marauders and Marines are common, and without the proper tactics they can be devastating. Until 4:50 into the game (when Concussive Shells can finish) you can harass with Stalkers. Using a Probe to poke the opponent's ramp for the unit composition is a lot safer. In order to survive early MM pushes you need the right mix of Zealots and Sentries, with a few Stalkers in the back (perhaps a ratio of 6-4-3). Use Force Field to either cut Terran's army in half and trap them for your Zealots, or deny them entry to your bases(particularly on ramps).
 Robotics Facility
The Robotics Facility is the most common next step, and getting an Observer to make sure cloaked Banshees cannot kill off your Probe line. If not going for a Robo before you expand, such as the 1 Gate Expand you should get one Robo up whenever you feel it is safe to do so. Robotics is the tech path with the most advantages against Terran. Immortals do high damage to Armored units and receive low damage from Terran heavy hitters (Marauder, Siege Tank). Observers can be used to scout the opponents production. Be careful when you move your observer since it's easy to see the ripple of the Observer if you fly into your opponents vision (directly over their army or buildings). This also allows for quicker Colossus tech in the mid game.
 Common Openings
 Mid Game
Surviving into the mid game Terran now have a few different strategies. You will have to learn how to recognize and deal with them accordingly. Scouting becomes very vital at this point in the game. By this stage you have to have your tech support set up. Gateway units are insufficient to deal with bigger groups of Marines and Marauders (especially with quick upgrades).
- MMM - The most common composition by far. Almost every pro game uses this composition. Colossus tech with upgrades for Gateway units is a safe alternative while transitioning to Chargelot/Archon.
- Marine-Tank - A force that is hard to attack head on. Primarily used for early 1 or 2 base all-ins. Warp Prism or Stargate are possible alternatives. Psionic Storm with Chargelots is very good against this composition.
- Thor-SCV - Normally used for quick all-ins hoping to outrepair any available DPS, although occasionally used for later attacks. Thor builds usually require more Immortals and a lot of focus fire or other way to deal with repairs. Void Rays have been known to work, but be careful of splash.
- Banshee / Raven mixes - Generally this comes in the form of an all-in. If you can, place cannons and create a balance of Stalkers Sentries and Zealots.
- Marine, Marauder, Ghost - Sentry force fields are key to defend this as well as splitting your Sentries to make ghost's EMPs as inefficient as possible by doing so, delay enough to gather up some forces to be able to combat the upcoming army. Later, tech to Colossus.
In the mid game, be extra careful to place Observers denying any Medivac drops, especially if you are ahead. This is a common way for the Terran to delay your forces in order to recover.
- Early probe harass against Terran SCVs while they are building structures is effective and extremely annoying to the Terran player.
- Terran wall-in is effective against Gateway units in the early game. Back off once shields are depleted to let them regenerate, then return to keep up the pressure. Even if you do not break the wall-in, it is worth it to keep your opponent contained. Using a Void Ray to harass the wall is also effective.
- Once the Terran gets Siege Tanks, you will need Immortals or Stargate units to break the fortification. Stalker Blink can be used to bypass Siege Tanks in some cases.
- As in all Protoss matchups, proxy Pylons can be used to warp in Gateway units closer to your opponent's base.
- Place Pylons throughout your main to provide vision against very common Terran Medivac drops.
- Keep important tech structures away from edges of your base to avoid losing them to Medivac drops.
- An early Observer is important to prevent Banshee harass. Photon Cannons on the mineral line are useful if you can afford them.
 Additional Notes
- The EMP ability of Terran Ghosts will instantly remove 100 shields from all units in the area of effect, and will temporarily de-cloak Dark Templar. This makes Ghosts a high-priority target.
- Raven's Point Defense Drone will absorb 20 attacks from Stalkers. If you have a Stalker-heavy army, Ravens are a high-priority target.
- Getting an armor upgrade along with a Sentry guardian shield helps Zealots against Marines; 1 Zealot base armor, +1 for armor upgrade, +2 for shields -- an upgraded Marine attacks for 6 damage -- it takes three times more shots for an unupgraded Marine to kill a Zealot under these circumstances.
- The most common endgame composition is Chargelot/Archon/High Templar/Colossi.
 Unit Roles and Counters
- Zealots: Good against non-upgraded, non-microed Marines and Marauders. Concussive shells slow Zealots down so it is important to use Sentries' force fields to eliminate the mobility of Terran infantry. The Zealot Charge ability in mid-game is important to eliminate Terran ranged advantage.
- Stalkers: Can be effectively used against Marines in the early game due to superior range and speed, and absorbing hits with their shield which can be regenerated, but this advantage fades when Stim is upgraded. Weak to Marauders. Blink can be used in combination with a flying unit to bypass Terran fortifications. Small groups of Stalkers are also used to deny Medivac drops.
- Sentries: The Sentry's Guardian Shield ability reduces damage from Marines (33%) and Marauders (20% or 10% vs Stalkers). Force Fields are useful to block chokes and ramps and to split the Terran army. Units trapped by force fields cannot retreat from Zealots.
- Observer : Observers are useful for monitoring enemy army positioning. A well placed observer can show when the opponent unsieges, which is the perfect time to engage. Observers can also be used to spot incoming drops.
- Immortals: Strong against Marauders and Siege Tanks due to Hardened Shields and armor damage bonus. Poor against Marines. Large numbers of Stimmed Marauders with focus fire can overcome Immortals. Immortals are also very effective against Thors. Without upgrades and micro, one Immortal can almost kill one Thor.
- Colossi: Counters Marine/Marauder bio-balls, but very vulnerable to Vikings.
- High Templar: Powerful against Terran bio-balls and sieged Tanks. Templar spells, or lack thereof, often determine the outcome of lategame battles.
- Dark Templar: Useful since Terran often do not produce Ravens. Keep them separated and be prepared to run out of the range of scans to keep them alive and waste Terran Orbital Command energy. Dark Templar also force Terran to stay in their base until they either save up scan energy or get a Raven, which can allow you to safely tech or expand.
- Archon: Works well in combination with Charge Zealots ("Chargelots"). Archons' high shields can absord a large amount of damage while the unit does powerful splash damage. However, EMP can allow any Terran composition with ample Ghost numbers to annihilate Archons. In late game situations, Archons' anti-air splash can be useful for killing clumped Vikings quickly.
- Phoenix: Counter to Vikings and Medivacs. Good use of Graviton Beam against Siege Tanks can turn the tide of a battle.
- Void Ray: Good counter to Siege Tanks, but vulnerable to Stimmed Marines due to slow movement speed.
- Carrier: Very powerful unit if the Terran lacks armor upgrades. Carriers are weak against Vikings, but only if there are superior numbers of Vikings compared to Carriers. Vikings can be produced faster than Carriers, so make sure you support the Carriers with Stalkers and Sentries. Almost never used in competitive play because of the role overlap with the Colossus.