I am working on a SharePoint list which I did not create and where new items are added through a form (DispForm.aspx). In the list itself, many columns are duplicated with similar names and since I did not develop the list myself it is difficult for me to understand to which field/column in the list every text box, combo box, etc. in the form is pointing to. I have tried to edit the form in SharePoint designer 2013, but I get this error message:

After clicking OK multiple times, the page then opens, but I only see the HTML code and I have no way to see the fields that the form points to.

Any help will be greatly appreciated.

Thank you and best regards.