how to turn off MacBook air screen while using external monitor

Hi,

Using the built-in keyboard and trackpad

You can dim your Macbook's screen all the way down (just press brightness down repeatedly) and then connect the MacBook Air to your monitor with HDMI or USB-C. Your screen will be off but the keyboard and trackpad built into the MacBook Air will still be on and available.

If you use external keyboard and mouse, just lid the screen.