Form field conditionnal display

In this snippet the text field mycustom is only displayed if the value of the select field myoptions is custom

$options = drupal_map_assoc(array(100,200,'custom'));
$form['myoptions'] = array(
  '#type' => 'select',
  '#options' => $options
);
$form['mycustom'] = array(
  '#type' => 'textfield',
  '#states' => array(
    'visible' => array(
      // Jquery selector
      ':input[name="myoptions"]' => array('value' => 'custom'),
    ),
  ),
);
<drupal7>

Display errors - WSOD

The bootstrap override the values you could set to display error ad the reporting level. To be 100% sure everything is printed out when something fails in a blank screen (wsod), make index.php like this:

Properly render a field

Drupal offers a few (sometimes complicated...) solutions to output a field. Lets take an example with a date field.
To display the full rendered field: field_view_field()