I'm building a form module. One of the early fields is a set of radio buttons. By default the first button is selected. Next I'll have a series of select boxes. One needs to be visible, the others invisible. Then as the user selects a different radio button I want different select boxes to show or hide. How can I hide the field and label by default and show it later dependent upon which radio button (or another select box option for that matter) is chosen?
You can use the #states property to achieve that. The #states property can be applied to all Form API elements.
Here's the documentation link with an example.