how do i delete photos from my mac but keep them in icloud?

You don't. If you are using iCloud Photos your computer and iCloud have the same photos. However you do have the option of your Mac having an optimized (smaller) version of the photos. This saves a tremendous amount of hard drive space. To make this choice, open Photos, select Preferences and click on the iCloud tab. Select Optimize Mac Storage.

This is controversial around these parts, but I recommend you get your photos out of the Apple ecosystem and store them as one photo per file on multiple external storage devices. For long term storage, don't rely on usb memory sticks nor external ssd drives. LoC does not have data on the reliability of usb memory sticks. Apple seems to be saying do not store photos on external ssd drives. One device should be off site. Using a cloud service is fine. Remember a lot of cloud services are sync services. That is should the file be deleted on the local device, the photo will be deleted in the cloud. For this reason, I never install the cloud service software on my computer. I use only file upload. For those who complain, my assessment is that the apple Eco-system doesn't match Library of Congress's requirements for digital archiving.

informative video:
http://www.digitalpreservation.gov/multimedia/videos/personalarchiving-photos.ht ml

printed version
http://www.digitalpreservation.gov/personalarchiving/photos.html

General information on digital preservation:
http://www.digitalpreservation.gov/personalarchiving/

R