how can i delete all photos from Mac Book but still keep them in I cloud. I need space in computer.

However - if you turn off iCloud photos on the Mac, it is possible to also delete in the photos app, wihout deleting in iCloud.

However, if you want to delete all the pictures, then deleting the library as proposed by Keith is the best way. I would suggest backing it up to an external drive first. Managing photos purely in iCloud is not the easiest way of dealing with them. You also need to make sure you have a backup in any case. Once your photos are only in iCloud they are the only copy you have.

You could even move the library to an external drive, and run it from there if you wish.

See

https://support.apple.com/en-gb/HT201517

Only by deleting the library. If you delete them from Photos, they are deleted in iCloud. iCloud is a *syncing* service, not a *storage* service.