IDM downloads only a commercial video that is shown before a main video. How can I download the main video?

This probably happens because site that you are trying to download video from uses another protocol for main video. It can be some protected protocol for example.

IDM will not download protected streams with copy protection measures like encryption. According to international and US laws, we cannot circumvent copy protection measure implemented for the protection of copyrighted streams.

If IDM does not show its download panel for some videos, most likely this web site uses a protected download protocol that IDM does not support for legal reasons (read details).

Also maybe this protocol is not yet supported by IDM. We plan to increase a list of supported protocols in the future. You can report exact web pages with such videos to us.