The tax is paid twice per year. It depends on the fuel economy, is different for diesel and gasoline vehicles and depends on weight. Owners of EVs also pay the tax.
For more information, see: https://www.iea.org/policies/3013-green-owner-tax?country=Denmark&qs=den IEA/IRENA Global Renewable Energy Policies and Measures Database © OECD/IEA and IRENA, [November 2020]