check every nth post wordpress

check-nth-post.php

<?php
// $the_query would just be a standard WP_Query
if ( $the_query->have_posts() ) : 

$counter = 1;

while ( $the_query->have_posts() ) : $the_query->the_post();

if ( $counter % columns == 1 ) {
    // do something at the start of your 'row'
}

// some content

// Check if $counter/$columns is equal to an integer
if ( is_int( $counter / $columns ) ) {
    // do something at the end of your 'row'
}

$counter++

endwhile;

// Reset WP_Query() data
wp_reset_postdata();

endif;

?>