You are offside if you receive a pass from one of your teammates and there are less than 2 defenders/opponents ahead of you
"Common mistakes":
1. You could be offside even when there's a defender ahead of you, there must be always 2 players including goalkeeper (this is really a strange situation, could happen after a corner kick mostly)
If the player with the ball decides to pass to the one at the left instead of shoting it would be offside since there's only 1 opponent ahead of him
2. You are not offside if you are coming from your own field's half the moment the pass is made
3. You are not offside if the pass is going backwards, doesn't matter if you are ahead of every defender
4. You can't be offside with your arms because it has to be with a part of the body you can score with
5. You are not offside if the "pass" is coming unintentionally from an opponent
Dude, like I said, I was born in Spain and have been watching football since Barça signed Rijkaard and Ronaldinho till I was 20 and lost interest in the sport because it was too "prehistoric", thank god they finally opted for technology with the VAR, it is just stupid how Maradona gave Argentina a world cup with a hand goal or how Henry made France go to World Cup with a hand play, riged sport favoring bigger teams
Now they need to stop the clock every time the game is stopped with a foul, corner kick, goal kick... make the game 60 min then because 90 min would be too much