To prevent zergling rush or reaper rush, make sure you scout your opponent early in the game. For terran, build a supply as soon as you get your 9th scv, then scout with that scv after finishing the supply. Protoss, make your 9th probe and build a pylon with it and then scout. Zerg, make your 10th drone (and make your overlord after the 10th drone) and then scout with that drone as soon as it finishes. If you see a pool that is at least 50% complete by the time you see their base, its a zergling rush so prepare for it. If you see terran with a refinery and an early barrack going for tech add on, its a reaper rush.
The first wave of zergling rush can be easily killed with your workers. When they come for your workers, make sure you get a good surround on those zerglings before attacking or 6 of them could take out your entire base. Make sure you keep producing units at barracks in case more zerglings arrive. Also when you're playing against zerg, make sure you make your entrance 1 block by building if your protoss, or completely shut it off if you're terran. This will stop any more zerglings from coming in after the first wave.
Reaper rushes are really annoying if you're protoss. As for zerg, you don't need to do anything other than a queen and a couple zerglings (but if he's massing reapers, get speedlings or put up a crawler or two). As for terran, start getting marines right away and mauraders if you can (reapers barely do any damage against mauraders). As for protoss, as soon as you scout the reaper rush, you need to place down a cybernetics core as soon as possible since only counter against reaper rushing is stalkers. When cybernetics completes, chronoboost your barrack and get a stalker out asap.
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