how do i add movies and tv shows to my iPod nan 8th gen

The current iPod nano is 7th gen. Apple changed the color selection in 2015, but it's technically still the 7th gen iPod nano; some vendors are calling it "8th gen."

Your iPod nano cannot play purchased TV shows that are HD, because the video resolution is too high. If you purchased and downloaded the HD version, you should be able to download the SD version at no cost, which should sync and play on your iPod nano.

Go to the Purchased screen in iTunes Store

Download your past purchases - Apple Support

Set it to show TV Shows and All. At the bottom right corner of the screen, there is a checkbox for Download HD When Available. If that is currently checked, uncheck it. Then, find the TV shows you want to add to iPod, and click its cloud with down-arrow symbol. If you only had the HD version, the SD version should download, and you'll have both version.

Please post back if this is not the reason for your problem.

You don't "change" it. You download it (the SD version) in addition to the HD version, so that your iTunes library has both versions. When you sync your iPod, iTunes uses the SD version. When you play it in iTunes on the computer, iTunes uses the HD version.

How do i get the SD version i got the HD version

HOW is described in my first reply, above.