It is a good idea to show post excerpts in the month, category and author archives of your Wordpress blog. When a user visits an archive page he will only see the first few lines of the post and if interested may proceed to read entire article(s). This besides being more user-friendly will serve two goals, firstly more pageviews and secondly no duplicate content on your site.To control content on category page you have to edit the file category.php of your theme. For author page it is author.php and that for month page, archive.php.

If your theme doesn’t have separate category.php or author.php then you will have to edit archive.php. And if archive.php is missing then probably index.php as dictated by the the wordpress file hierarchy.

Let us take the following code as an example.

1 <?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>
2 <div class="post">
3 <small><?php the_time('F jS, Y'); ?></small>
4 <div class="entry">
5 <?php the_content(); ?>
6 </div>
7 <p class="postmetadata">Posted in <?php the_category(', '); ?></p>
8 </div> <!-- closes the first div box -->
9 <?php endwhile; else: ?>
10 <p>Sorry, no posts matched your criteria.</p>
11 <?php endif; ?>

Look at line 5. This shows the full post. Replace this line with

<?php the_excerpt(); ?>

This will show only the excerpt.