Q: how can I delete photos on iphone but keep them on ipad?

If they are logged on to the same ICloud then, whatever you delete, i think i will delete in the next. But you can delete it and send it to your ICloud.

If you are using iCloud Photo Library on both devices anything you delete from one will be deleted from the other and from iCloud. The only thing that you can do to reduce the storage space required on one or both devices while continuing to use iCloud Photo Library is to turn on Settings > iCloud > Photos > Optimize Storage on the device(s) where you want to reduce the space required.

As FoxFifth said

"If you are using iCloud Photo Library on both devices anything you delete from one will be deleted from the other and from iCloud."

Are they under the same ICloud account ?

suelyne wrote:

 

I want to delete them off my iPhone and store them on my iPad

You haven't given any information about whether you are using iCloud Photo Library. Another option would be to turn off iCloud Photo Library on your phone (and only on your phone) and turn on Settings > iCloud > Photos > Upload to my Photo Stream.

Note that I have not used Photo Stream since iCloud Photo Library became available and I am not certain what will happen if you transfer a photo from your iPhone to your iPad via Photo Stream and then delete the photo from your iPhone. The following has Photo Stream information but I don't see a clear answer to that so you would be well-advised to test with a new photo that you take for that purpose and don't care if it is lost. My Photo Stream FAQ - Apple Support

The only thing you can do is what FoxFifth said which is  "The only thing that you can do to reduce the storage space required on one or both devices while continuing to use iCloud Photo Library is to turn on Settings > iCloud > Photos > Optimize Storage on the device(s) where you want to reduce the space required."