I said "since you wished me happy birthday a while back..."! You DID DEFINITELY wish me a HB - that's why you only got a yellow and not a straight red
You do know more than you were letting on, that's good. You were dumbing it down for Fab Foodie, I see. Glad to know that you understand it "in theory"
Let's get to the nuts and bolts. The offside rule is simple but for one thing; how do you know if a player is active or not. That's the only difficult part and that's the part that's causing all the bother, because it leaves it open to interpretation. You might consider a player to be active when I consider him to be inactive. The rest of the rule is pretty self-explanatory.
As you (or someone else?) point out, it's easy for the ref or his assistants to make mistakes in enforcing the rule through poor observation, but it's not complicated with the exception of what I've outlined here.
A few other parts to the rule for good measure!
1. A player cannot be deemed to be offside whilst standing in his own half.
2. A player cannot be deemed to be offside if the pass he receives is not a forward pass.