When you open an article in on the Articles main tab, the main window may look like on the following screenshot. Note that the WYSIWYG pane is not displayed. The article tree covers all.

Double-click the article element you want to edit in the source pane, or use keys to select and edit an article element, and it will automatically create a split between the tree pane and the source pane (see below figure).

See also Edit more elements at once in source pane, About initiating and ending editing of article elements and End element editing in source pane.