Comments_link not displaying the correct wording

I am having a few issues with displaying the comments link on my site.

I am using my own child theme (with the TwentyFourteen theme), the code is directly copied from there.

<span class="comments-link"><?php comments_popup_link( __( 'Leave a comment', 'themename' ), __( '1 Comment', 'themename' ), __( '% Comments', 'themename' ) ); ?></span>
(obviously I have changed the themename part to my own theme’s name)

When I have no comments, I would like it to say “leave a comment” instead it is saying “0 Comments” How do I fix this.

Secondly, I don’t want the comments_link to show up on the single post page.

I figure I need to create some sort of if single loop, but I don’t know the correct code to add to the is single part.

<?php
        if (is_single()) :

else :
<span class="comments-link"><?php comments_popup_link( __( 'Leave a comment', 'themename' ), __( '1 Comment', 'themename' ), __( '% Comments', 'themename' ) ); ?></span>
        endif;
        ?>