how to hide all but one field when form is open

I think you've figured this out already. You have to configured a show/hide conditional logic in the form. 

https://www.jotform.com/help/316-How-to-show-or-hide-a-field-in-the-form-based-on-the-other-field-input-or-selection

https://www.jotform.com/help/297-Show-or-Hide-multiple-fields-at-once-using-form-collapse-tool

https://www.jotform.com/help/196-Hide-or-Show-Multiple-Fields-at-Once