There are if-then loops, forever loops and repeat until loops.
The repeat until loop so that you can describe the action that occurs when you want the given situation to stop.
There is no need to put a forever loop inside of another forever loop so it does not make sense.
They will know when to begin an action if you use an if-then loop or a wait until control.
No, because one will switch to the next costume when the space bar is pressed and the other will switch to the next costume with the spacebar not pressed.