once a player goes out he cant come back in. generally 3 subs per game, however ive seen more in friendly matches. clock keeps running no matter what. they sub in when there is a break in the flow. player checks in and then when the ball goes out of bounds, or there is a penalty, or anything really that stops the flow of the game, he joins the game. managers usually sub when there is an injury, player gets red or yellow carded, or late in the game for fatigue or tactical reasons.
the game is very simple. i look at our traditional american sports and wonder why do we need all the rules to make things complicated.