Systems with Intel's 7th generation processors or later do not have support for earlier versions of Windows. See here:
You will have to run Windows 10 Pro as the main OS. Although you could setup a Windows 7 VM on the side.
My laptop originally came with 7, upgraded to 10 a year or so ago, and would like to downgrade back to 7. Can this be done?
Yes if Windows 7 as preinstalled it is possible to reinstall it, if you've got Windows 7 Installation Media. What model of Dell is this and what Edition of Windows 7?