Modify Comment Form Input Fields & Text Labels.php

function wpsites_modify_comment_form_fields($fields){
    
	$fields['author'] = '<p class="comment-form-author">' . '<label for="author">' . __( 'Author', 'wpsites.net' ) . '</label> ' . 
	
	( $req ? '<span class="required">*</span>' : '' ) .
                    
	'<input id="author" name="author" type="text" placeholder="Real name, please, no keyword spamming!" value="' . 
					
	esc_attr( $commenter['comment_author'] ) . '" size="30"' . $aria_req . ' /></p>';
					
    $fields['email'] = '<p class="comment-form-email"><label for="email">' . __( 'Address', 'wpsites.net' ) . '</label> ' .
    
	( $req ? '<span class="required">*</span>' : '' ) .
    
	'<input id="email" name="email" type="text" placeholder="Preferably Linked To Your Gravatar.com Account" value="' . 
	
	esc_attr(  $commenter['comment_author_email'] ) . '" size="30"' . $aria_req . ' /></p>';
	
	$fields['url'] = '<p class="comment-form-url"><label for="url">' . __( 'Domain', 'wpsites.net' ) . '</label>' .
    
	'<input id="url" name="url" type="text" placeholder="Please Link To Your Own Domain" value="' . 
	
	esc_attr( $commenter['comment_author_url'] ) . '" size="30" /></p>';
	
    return $fields;
}

add_filter('comment_form_default_fields','wpsites_modify_comment_form_fields');

Add this code in wordpress theme functions.php