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, that IDM does not support for legal reasons. Please read here what to do in such case.
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.