For documents with fixed markup, such as forms in which the XML tags are not allowed to be
modified (only their text content), the possibility to edit the XML tag names can be
disabled/enabled with the 
Lock / Unlock the XML tags action available in Text
editing mode from:
Lock / Unlock the XML tags action available in Text
editing mode from:- The menu.
- The Source submenu from the contextual menu.
You can set the default lock state for all opened editors using the Lock the XML tags option in the Text preferences page.