You cannot block a shot after it starts descending toward the basket or touches the backboard. On offense, you can’t touch a live ball on or inside the rim.
Basketball is a non-contact sport in theory, though it is highly physical in practice. Players are allowed to play defense, but they cannot make illegal physical contact. 10 basketball rules
Once the offense crosses half-court, they cannot go back into the backcourt (unless the ball is tipped by defense). Doing so loses possession. You cannot block a shot after it starts
Each team has 5 players on the court at a time (point guard, shooting guard, small forward, power forward, center). Unlimited substitutions are allowed, but only during dead balls. center). Unlimited substitutions are allowed