how to delete photos on my iPhone but not in iCloud

There is no option to do that.

iCloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See: https://support.apple.com/en-us/HT204264

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.

Another way that photos may be on iCloud (if you don’t have iCloud Photos turned on) is via a backup of an iOS device. The only thing you can do with a backup is use it to restore a device or set up a new device. The contents of backups are not viewable at iCloud.com and if you delete a photo from a device it will not be included in future backups and will be lost.

If you have been using iCloud Photos and your photos are now in iCloud, but you want to remove the photos from the iPhone, the only way is to turn off iCloud Photos on the iPhone.

when your iPhone is no longer syncing with iCloud Photos, you can delete phots from the iPhone and they will remain in iCloud. However, you can no longer see the iCloud photos in the Photos.app on the iPhone and work with them. You will only see them on your other devices or on the iCloud web page www.icloud.com . The iPhone will only show you this page, if you request the Desktop site. It will be highly inconvenient, unless you do no longer want to use the iPhone to work with your photos.