If you are only going to remember one external keyboard shortcut for your iPad, remember ⌘. When you press ⌘ anywhere in iOS you are presented with an overlay showing all the keyboard shortcuts available there. So ⌘ is number one on the list. The rest of them are those you use for getting around between apps in iOS and taking screenshots.
⌘ H: Go to home screen.
⌘ spacebar: Search and open apps.
⌘ ⇧ 3: Take a screenshot.
⌘ ⇥: Switch between apps.