Adds a counter to check for the first post in a WP loop.

wp-loop-checks-first-post.php

<?php $count = 0; ?>
<?php if( have_posts() ) : while( have_posts() ) : the_post(); ++$count; ?>
<div class="post">
    <h1><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h1>
    <?php if (($count == '1') && (!is_paged())): // If it's the first post on the first page. ?>
        
        <?php if ( has_post_thumbnail() ) the_post_thumbnail(); ?>
        <?php the_excerpt(); ?>
        
    <?php else: // If it's any other post on any other page. ?>

        <?php the_content(); ?>
        
    <?php endif; ?>
</div>
<?php endwhile; endif; ?>