No. You can have one foot on the ground or no feet on the ground as long as the player has secured the ball. ie. A player can signal for a timeout in mid-air as long as he/she has secured the ball and signaled for the timeout before stepping out of bounds.
yes. it is where the ball is, not where the fielder is.
Yes. If a foul ball is caught by a defensive player before it hits the ground the batter is out.
To make sure that it is safe and nothing is one it that could injure a player.
Yes. there are no "crossing the plane" rules in basketball. Its always about where your feet are touching the ground or where they were last touching the ground.
It is essentially an out. If it is a fly ball the player catching it receives a put out. If a ground ball and the player throws the ball to first, the player throwing receives an assist and the player catching and tagging the base before the runner receives the put out.
No. A player is down when his knee or back touches the ground. If the ball carrier loses the ball before this happens then it is called a fumble, and both teams try to recover the ball and whichever team does gains possession. Yes. The only thing allowed to touch the ground without you being 'down' is your hands and feet. We even have interpretation questions on whether the wrist and ankles are part of the hand/feet to determine if a player is down. If the ball touches the ground, the player is considered down. This is why the ground cannot cause a fumble. If it was a pass and the WR was deemed not in control of the pass, it is known as a dropped or incomplete pass. The player has to have complete control, but the ball can touch the ground. This is determining the completion of the catch, though, not whether the player is down or not.
The batter is out, if they hit a ball in the air and a defensive player catches the ball before it touches the ground, in fair or foul territory. If a defensive player has the ball in control and touches a base before the runner, the runner is out. If a defensive player has the ball in control and touches the runner with the ball in their hand or glove, the runner is out.
If the ground causes a fumble the player is ruled down.