What's the zone?
The strike zone is probably the most difficult aspect of umpiring to master.
It takes a lot of games and a lot of practice. There are pitches that technically meet the definition of a strike that are balls and there are pitches that meet the definition of a ball that are strikes. You have to be able to know what's too high, what's too low, what's outside and what's inside.
There is an old saying, "perception is reality." The "perception" is not what you see, but rather what all of the players, coaches and fans see. Remember, only you and the catcher are behind the plate. Everyone else has a different angle. The pitcher can crank off a beautiful curve ball that breaks sharply, briefly enters the strike zone and goes down and out causing the catcher to go to his right and into the dirt to catch it. You call it a strike, which it is, and everyone starts screaming and yelling. The next pitch is a fast ball, mid thigh and two inches off the plate right into the catchers mitt. You call it a ball, which it is and everyone is screaming again. So, you learn to start adjusting. The secret is knowing how far to adjust and also keep it consistent throughout the game. The next game, depending on the pitchers, you may have to adjust again.