post fully opened when clicked category

a link to your blog would help 😉

the template-tag to show the whole post is:
<?php the_content(); ?>

the template-tag to show the excerpt is:
<?php the_excerpt(); ?>

the template-tag to show the title is:
<?php the_title(); ?>

if there’s a file archive.php in your template directory, you’ll find the above title-tag in this file. to show the whole post you have to add the “content-tag” right after the title-tag.

for example:

<div class="bla">
<h1><?php the_title(); ?></h1>
<?php the_content(); ?>
</div>

if your archive page (the page with posts from only one category, date, etc.) looks the same as your index page, you can copy this from your index.php.