A lot of Protoss players consider Zerg as the most difficult species to play against. There are several reasons. First, Zerg units are much faster than the Protoss units, so it is sometimes difficult to catch up with Zerg’s speed. Second, once defiler comes out from Zerg with four gases of resources (four bases) under his belt, it is very difficult to win that game due to defiler’s swarm and plague. Swarm allows Zerg units to have no damage when they are under the swarm, except short range units like zealot. Short range units can damage the Zerg units under the swarm with lesser damage. Plague decreases Protoss unit’s stamina all the way down to one, making Protoss units easy to kill. Because of defiler, Protoss professional players usually advise players to finish Zerg before he gets to the Hive stage and produce defilers. Third, Protoss has to think of multiple ways to block Zerg’s strategies because Zerg can come up with so many strategies: all hydra rush, mutalisks with scourge, lurker drops, et cetera.
A couple keys to consider when you are playing against Zerg. Firstly, you need to continuously harass Zerg either with your zealot, probe, or whatever you may bother with. It can be very dangerous for Protoss to let Zerg grow without any harassment because generally people say Zerg is strong against Protoss, Protoss is strong against Terran, and Terran is strong against Zerg just like rock is strong against scissors, scissors paper, and paper rock. Therefore, you should think to move few more steps further than what the Zerg is doing in order to win the game. For example, Zerg will make several bases in the beginning. Harass each base simultaneously with zealots with multitask and make the Zerg stumble a little.
Secondly, do not try to have a lot of bases (Nexuses) unless you are sure that you can protect them and that you are ahead of the game. What a lot of Protoss players try to do after their second base is they will continue to try to build more bases to acquire more resources. However, often times, those third and fourth bases are easily destroyed by the Zerg players. So, make sure that you have a full control of the game by moving your masses of units pressuring Zerg while you extend your bases.
Lastly, what you should consider important are the importance of the use of high templars and corsairs. These two units are the key essentials for success against Zerg. Templars are extremely useful when fighting against massive hydralisks. Its ROIs (return on investments) are massive. Also, pressuring Zerg’s overlords with corsairs all the time during the game can really bother the Zerg players because fewer overlords mean less units are allowed to spawn. By killing the most number of overlords, you can halt Zerg from growing and gain a lot of time yourself for development and battles. Corsairs are also necessary to combat mutalisks and scourges.