Play proceeds clockwise. You must follow suit if you can.; If you cannot follow, you may play any card — this is how penalty cards get discarded onto other players.; The highest card of the suit led wins the trick, and that player leads the next.
Why people argue about this
People often assume that if they can follow suit, they must always do so, leading to arguments about whether a player should lead with their highest card of the suit just because they could have followed instead of playing another card entirely. The rule specifically allows for discarding cards when you can't follow suit, which some players might overlook or misinterpret as mandatory following.
