"OFFSIDE REF !! "
On the 29th October 2003 FIFA, Football's world governing body, published a directive "..to ensure uniform interpretation of Law 11: Offside."
The aim of this decision is to respect the Laws of the Game and to protect attacking play intended to lead to a goal, which is the ultimate objective in football. This is not a change to the Laws of the Game as this interpretation adheres entirely to the original wording of the Law.
LAW 11: A player in an offside position is only penalised if, at the moment the ball is touched or played by one of his team, he is, in the opinion of the referee, involved in active play by:
INTEFERING WITH PLAY 
If the attacker receives the ball either directly or courtesy of a flick on from a team mate or a defender then he/she is offside.
This is because by receiving the ball he/she will be interfering with play.
If the ball is cleared then that phase of play is over. That means that the attacker's initial offside position no longer stands.
If the ball is then played into the box again the attacker's new position is the one that counts.
FIFA SAY:
“interfering with play”
PLAYING OR TOUCHING a ball passed or touched by a team-mate.
INTERFERING WITH AN OPPONENT
If an attacker interferes with an opponent by either preventing them from playing or being able to play the ball, then he/she is offside.
One way of doing this would be to block the goalkeeper or get in his/her line of vision as you can see above.
FIFA SAY:
"interfering with an opponent”
PREVENTING an opponent from playing or being able to play the ball. For example, by clearly obstructing the goalkeeper’s line of vision or movements.
Making a gesture or movement while standing in the path of the ball to DECEIVE OR DISTRACT AN OPPONENT.
GAINING AN ADVANTAGE 
Lastly if the ball is played into the penalty area and the ball rebounds off either a post, the crossbar or an opposing defender then the attacker is offside.
This is because he/she has gained an advantage by being in that position.
FIFA SAY:
“gaining an advantage by being in that position”
PLAYING A BALL that rebounds off a post or the crossbar having been in an offside position.
PLAYING A BALL that rebounds off an opponent having been in an offside position.
SO THAT IS CLEAR THEN
FIFA ALSO SAY: in the same directive, in case you are still in doubt
"As stipulated in the Laws of the Game, the referee’s decision is final!"
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