Can someone explain succinctly exactly what constitutes a foul?
When you play the man and not the ball.
Broadly, but it’s a very grey area now. Playing the ball is not enough to be a legal tackle. If you play the man and the ball, or even in some cases ball only, it can still be a foul if the tackle is felt to be careless or reckless or endanger the safety of the opponent. Which is why the 2 footed lunge where the tackler misses the opponent and gets up making the symbol of the ball with their hands is an automatic red card because of the danger rather than the outcome.
Hence why the foul for the freekick was a wrong decision, but not a badly wrong decision. He got the ball and he got the man, however did he get ball first, man first, ball and man together? In the past it didn’t matter as long as you got the ball somewhere in there – Tony Adams made a career of it 😉
[edit – I do still have my Ref’s badge, whistle and cards in the loft somewhere]