Who Covers Which Base
Who covers which base on every ball in play and every steal.
How it actually works. On a steal of second the coverage is decided by the hitter: with a right-handed batter the shortstop generally takes it because the ball is more likely to be hit to his side and the second baseman is the one who can leave, and with a left-handed batter it flips to the second baseman. The pitcher and catcher are told which before the pitch by a sign from the middle infielders behind the hitter's back. On a ground ball to the first baseman the pitcher covers first, breaking toward the line and then up it, receiving the ball a step before the bag. On a bunt with a runner on first the second baseman covers first and the shortstop covers second. On the wheel play with a runner on second the shortstop covers third.
Why it is done, and when it is wrong. The rule underneath all of it is that the fielder who is least likely to have to field the ball is the one who leaves to cover the base. That single idea will get you the right answer on almost any play you have not memorized, and it is why coverage flips with the batter's handedness rather than staying fixed. The corollary is worth saying too: whoever is not fielding the ball and not covering a base is either backing up a throw or getting into a relay line, and there is no play where anybody stands still.
What people get wrong about it. The pitcher covering first is the most commonly butchered routine play in baseball at every level, and the reason is the route. A pitcher who runs straight at the bag arrives moving across the line of the throw and has to catch and tag in one motion, and a pitcher who runs to the line first and then turns up it arrives moving with the throw. The second version is easy and the first one is how you get a collision and a dropped ball.
At Little League and travel ball. Coverage is the single biggest gap between a youth team that has practiced and one that has not, and it does not require athleticism. A 10U team that knows who covers second on a steal, both ways, is ahead of most of the field.
Batter's Eye: look up anything in baseball