I am trying to connect my MacBook Air M2 to an external monitor via an HDMI adapter. The external screen appears for 1 second when I switch it all and then goes black screen.

Hello anouchka1111,

We would like to see how we can help you with connecting to your external monitor. Follow the steps in this link for help with external display issues: If your Mac doesn’t recognize an external display - Apple Support

1. "On your Mac, choose Apple menu  > System Settings, then click Displays  in the sidebar. (You may need to scroll down.)
2. Press and hold the Option key to make the Detect Displays button appear in place of the Night Shift button, then click Detect Displays.
3. If your Mac still doesn’t recognize the attached display, try any of the following:

• Unplug the display from your Mac, then plug it back in.
• Make sure the cable you are using is securely connected to the correct port.
• Make sure you are using the correct cable to connect your display. 
  • Tip: To get more detailed information about the type of video display your Mac supports, check the tech specs for your Mac: Choose Apple menu  > System Settings, then choose Help in the menu bar. Choose [your Mac’s name] Specifications, then scroll down to Display Support or Video Support (depending on your Mac).
  • Make sure you are using the correct adapter, if needed. See Apple Support article: Adapters for the Thunderbolt 4, Thunderbolt 3, or USB-C port on your Mac.
  • Put your Mac to sleep and then wake it
  • If you are using a third-party display, check the included documentation to ensure you are connecting the display according to the manufacturer’s guidelines."

Should you need further assistance, reach out to Apple directly at: Get Support - Apple

Cheers!