Insert and overtype modes
If you have made the Status bar visible it will display whether you are in insert or overtype mode as you type. If you are in insert mode, you will see the word "Insert" in the Status bar and any text you paste in will push aside text on either side of the cursor. If you are in overtype mode, you will see the word "Overwrite" in the Status bar and any text you paste in will overwrite existing text to the right of the cursor.
You can also tell which typing mode you are in by examining the article cursor. Clicking any text in insert mode displays the cursor as a thin vertical line ("|") between letters, whereas in overwrite mode it becomes a blue bar highlighting the letter to the right of the cursor like this.
Pressing the Insert key toggles between the two modes.
Note that when the Tree pane has focus, pressing the Insert key inserts a new node as the child of the current node.