how to delete photos on your mac without deleting them off your phone

How are you syncing the devices?

Are you using iCloud Photos on both devices with matching Apple ID? If so, then you cannot do what you want, as the whole purpose of iCloud Photos is to keep the same set of photos (and videos) on all of the devices connected to the same Apple ID.

If you are using some other sync mechanism, please tell us which one. Examples could include My Photo Stream or USB cable connection with Image Capture or iTunes.