How Can I Make Dynamic Search Form While Converting Html To WordPress?

In your index.php, you should have something a bit like:

<?php if ( have_posts() ) : ?>

    <!-- HTML -->

    <?php while ( have_posts() ) : the_post() ?>

        <!-- Post content -->

    <?php endwhile ?>

    <!-- HTML -->

<?php endif ?>

You can extend this code to say “if no posts and is search, display message”:

<?php if ( have_posts() ) : ?>

    <!-- HTML -->

    <?php while ( have_posts() ) : the_post() ?>

        <!-- Post content -->

    <?php endwhile ?>

    <!-- HTML -->

<?php elseif ( is_search() ) : ?>

    <!-- No posts and is search, show message -->

<?php endif ?>