How do I delete Certain photos on device , but not on iCloud?

You don't, sorry.

Using iCloud Photos, if you delete a picture on your iPhone, that picture is deleted also on iCloud (and viceversa).

You can transfer some (or all) pictures to your computer using this procedure:--> Archive or make copies of the information you store in iCloud - Apple Support

When done (your selected pictures are safely stored into your computer) you can delete what you want from your iPhone.

Regards

GIulio

You can’t. That is not how iCloud Photos works. It is a syncing service only, not off device storage. If you delete photos on your phone, they are deleted in iCloud.

Read ProustGiulio’s post about archiving photos. If you don’t have a computer, you need to find another cloud solution for storing photos, such as Google Photos.

If you delete photos on a device they are also deleted from iCloud.

Did you read what I wrote?