The one rule everyone gets stuck on

Offside, on the board

A player is in an offside position if they're in the opponents' half and closer to the goal line than both the ball and the second-last opponent when a teammate plays or touches the ball. Everything below is the same pitch, attacking left to right. The amber line is the one to watch.

Source: IFAB Law 11
Generated top-down football pitch with goals
Offside line
Passing teammate in a red jersey
Highlighted runner in a red jersey
Support runner in a red jersey
Second-last defender in a blue jersey
Goalkeeper in a yellow jersey
Football at the passing teammate's feet
Attacking team
Defenders
Goalkeeper
Ball
Offside line