How to add SubTitle right below Title in Post

Hi,

I tried to add SubTitle right below Title in Post but couldn’t get it to work.
I installed WP SubTitle plugins, I then follow the instruction to modify postslider.php with the following code but still have no luck. Could you please show me what I did wrong?
Thanks,
– JN
$meta .= “<span class=’subtitle-meta’>”. the_subtitle() .”</span>”;