Hi @Tiziano
I see now that the keywords are already assigned to the template here: https://github.com/pkp/ojs/blob/master/pages/article/ArticleHandler.inc.php#L121-L123.
If you would like to have the subjects displayed as well you would need to do the same for subjects.
Then you can add the code like this to your template article_details.tpl, e.g. here ojs/article_details.tpl at ojs-3_0_2-0 · pkp/ojs · GitHub :
{if !empty($keywords[$currentLocale])}
<div class="item keywords">
<h3 class="label">
{translate key="article.subject"}
</h3>
<p>
{foreach from=$keywords item=keyword}
{foreach name=keywords from=$keyword item=keywordItem}
{$keywordItem|escape}{if !$smarty.foreach.keywords.last}, {/if}
{/foreach}
{/foreach}
</p>
</div>
{/if}
S. also this post: Tag Cloud plugins OJS 3.0
And appropriately for subjects, if wished.
Best,
Bozana