I accidentally push something on screen to make the keyboard move up to the middle of the screen. It turns out this gets right in the way of most windows that I need to type in. Can anyone tell me how to put the keyboard back at the bottom of the screen? Thanks!

It sounds like you might have the split keyboard turned on. Press and hold the keyboard icon in the lower right corner of the keyboard. When the menu pops up, select "Dock". That will move the keyboard back to the bottom of the screen.

You can turn this off with: Settings -> General -> Keyboard ->Split Keyboard -> Off

If this (or other odd behavior) happens by accident again, you may have inadvertently used a four-finger swipe shortcut. I believe swiping with four fingers in the down direction should also move the keyboard back to the bottom, while swiping up moves it to the middle.