A family of Microsoft word processing software products for creating web, email, and print documents.
In addition to Doug's explanation, you should be aware that turning the Design Mode button on or off has no relationship at all to whether the form is protected.
If you're using legacy form fields, then you must click the Restrict Editing button, check the box in the "Editing restrictions" area and select "Filling in forms", and click the "Yes, Start Enforcing Protection" button.
An alternative to this sequence is to go to File > Options > Quick Access Toolbar and add the Lock button (from the All Commands group) to the toolbar. That button toggles form protection on and off.
If you're using content controls, you should not go to the Restrict Editing pane or use the Lock button. Instead, select the entire form, click the Group button on the Developer tab, and click Group in the menu.
One other hint: You can use the Tab key to jump from one content control to the next, except that you cannot tab out of a Rich Text content control -- it just inserts a tab character.