The Start, or Modern, screen doesn’t have a taskbar, so when you run Modern apps you might lose track of which apps are running. There are a few ways to check which Modern apps are running and switch among all running apps.
One way of seeing which Modern apps are running is to press Alt + Tab. A dialog displays showing thumbnails of all apps running from the Modern screen. As long as you hold down Alt + Tab, the dialog stays on the screen. To cycle through the open apps, hold down the Alt key and press the Tab key. Each time you press the Tab key, the next app is selected on the dialog. When you get to the app to which you want to switch, release both keys.