i want add conditional for add category in wordpress

If I understood your question correctly, then something like this might do the trick.

// html markup for the selects
<select name="main_cat">
  <option value="A">A</option>
  <option value="B">B</option>
</select>

<select name="sub_cat_for_A" class="sub-cat-select hidden">
    <option value="C">C</option>
    <option value="D">D</option>
    <option value="E">E</option>
</select>

<select name="sub_cat_for_B" class="sub-cat-select hidden">
    <option value="F">F</option>
    <option value="G">G</option>
    <option value="H">H</option>
</select>

If you change your mind and want to use a taxonomy from WordPress, you can change the markup to this,

<?php
$main_categories = array();
$sub_categories = array();
$cats = get_terms(array(
  'taxonomy' => 'your_taxonomy',
));

if ( $cats && ! is_wp_error( $cats ) ) {
  foreach ( $cats as $cat ) {
    if ( $cat->parent ) {
      $sub_categories[$cat->parent][$cat->term_id] = $cat->name;
    } else {
      $main_categories[$cat->term_id] = $cat->name;
    }
  }
}
?>

<select name="main_cat">
  <?php foreach ( $main_categories as $id => $name ) : ?>
    <option value="<?php echo $id; ?>"><?php echo $name; ?></option>
  <?php endforeach; ?>
</select>

<?php foreach ( $sub_categories as $main_cat_id => $sub_category_options ) : ?>
  <select name="<?php echo "sub_cat_for_{$main_cat_id}" ?>" class="sub-cat-select hidden">
    <?php foreach ( $sub_category_options as $id => $name ) : ?>
      <option value="<?php echo $id; ?>"><?php echo $name; ?></option>
    <?php endforeach; ?>
  </select>
<?php endforeach; ?>

Then you just need some jQuery (vanilla javascript could be used also) to watch changes on the main select and to show the related sub-select.

jQuery(document).on('ready',function($){
  $('select[name="main_cat"]').on('change',function(event){
    var selectedMainCatId = $(this).value();
    // hide other sub cat selects
    $('.sub-cat-select').addClass('hidden');
    // show correct sub cat select
    $('select[name="sub_cat_for_'+selectedMainCatId+'"]').removeClass('hidden');
  });  
});

There might be room for improvement on the jQuery example, but I’m sure you’ll get the basic principle from it.