How to insert a text field inside boolean (single on/off) field in node form
You could implement hook_form_alter() and add a suffix or prefix to your field. The suffix and prefix may contain html.

    $form['terms'] = array(
      '#prefix' =&gt; '&lt;div class="my-class"&gt;Something before the field&lt;/div&gt;',
      '#suffix' =&gt; '&lt;div class="my-class"&gt;Something after the field&lt;/div&gt;',
    );


For a detailed instruction how to write a module that implements hook form alter see this blog post.

There is also a module that can add markup widgets to forms. Checkout Markup.