Nested if elseif to ger content slider and featured image help need

Hi, i am noob in wordpress, but bit understand some WP’s functions.

I am modifying a slider, defaultly it shows featured image or video, if both not exist, it shows blank, which upsets me and force me to upload a featured image.

so i want another function to show some selected part of my post content instead blank without removing formatting unlike excerpt. please advice me.

my mind says, if i give attribute in div , and that div shows in slider, it will be fantastic, my slider shows up to 640×350

and here my code

<?php $main_video=get_post_meta($post->ID, ’embed’, true); ?>
<?php if ( $main_video ) : ?>
<div><?php echo get_post_meta($post->ID, “embed”, $single = true); ?></div>
<?php else : ?>
<?php the_post_thumbnail( ‘content_image’ ); ?>
<?php endif; ?>