Are you sure you know when there is a goal and when there isn't? When there is offside, penalty, or indirect foul? Do you know who is responsible for calculating injury time or how assistant referees show players' fouls?
If yes, you don't need to deal with the Alphabet of Football Rules anymore. You are a perfect connoisseur of the sport and it's not worth wasting your time with such things.
But if you have doubts, if you have questions and if you don't understand why certain things happen the way they do, then immediately get the Alphabet, study it well, and have it with you next time you argue whether it was a penalty or not.