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.
NOTE: On a touch screen, swiping repeatedly from the left cycles through running Modern apps.
You can also move the mouse to the extreme, upper, left corner of the Start screen. A thumbnail of the last active Modern app displays. Click on it to activate that app.
Notice that there are partial outlines of boxes below the thumbnail. If you move your mouse down to those boxes, a sidebar displays thumbnails of the open Modern apps. Click a thumbnail to activate that app.
This sidebar also displays on the Desktop.
Clicking the Start button at the bottom of the sidebar also seems to take you to the last active app, unless you are on the Desktop. In that case, it takes you to the Start screen.
You can also simply launch a Modern app again to activate it. If the app is already running, it will become active. To toggle between the Start screen and the last Modern app you ran, press the Windows key.