PHP container

I think there are two ways of going about this. The first method is preferred because you don’t clutter up the HTML with empty DIVs. It involves filtering the post classes. Hopefully your theme follows best coding practices and includes the post_class() function. You can add the following to your theme’s `functions.php and it should check each post for the existence of a subtitle and add the appropriate class name to the post.

add_filter( 'post_classes', 'kia_add_subtitle_post_class', 10, 3 );
function kia_add_subtitle_post_class( $classes, $class, $post_id ) {
	if( function_exists( 'get_the_subtitle' ) && get_the_subtitle( $post_id ) ) {
		$classes[] = 'has-subtitle';
	} else {
		$classes[] = 'no-subtitle';
	}
	return $classes;
}

From there you can now style the two different post divs differently.

.has-subtitle h1.title { margin-bottom: 0 ; }
.no-subtitle h1.title { margin-bottom: 1em; }

I don’t know the markup of your theme so it’s up to you to get the right selector, but that’s an example.

Alternatively, you could do a similar check to determine if the post has a subtitle and if it does, display the subtitle, and if it does not, add some empty markup and style that.

if( function_exists( 'get_the_subtitle' ) && get_the_subtitle() ) {
	the_subtitle( '<h3 class="the-subtitle"', '</h3>' );
} else {
	echo '<div class="spacer"></div>';
}