Instead of deleting a form, you can make it inactive to save it in your form library for later use or to be copied and modified.
To make a form inactive, please got to the Evaluation page under Admin Tools. If you are having trouble locating the page, visit this help page.
Click on the Details icon of the desired form.
The Form Details page will appear. Uncheck the Active box at the top of the page by clicking the box.
Click Save to complete the process.