IDM starts downloading video automatically when I start playing it in a browser. How to avoid this?

You need to open "Options→General" IDM dialog and press "Edit" near "Customize IDM download panels in browsers".

Open 'Customize IDM Download panels in browser' dialog

Then enable "Don't start downloading from web-players automatically" option (arrow 1 on the image) and press "OK" (arrow 2 on the image).

Enable 'Don't start downloading from web-players automatically; option