How to make a page show posts only from specific categories without editing php files

Question

You can use WP_Query to lists posts with excerpt from specific category. Here is sample code:

<?php 
$args = array(
    'posts_per_page'   => -1,
    'cat'              => 7,
    'orderby'          => 'post_date',
    'order'            => 'DESC',
    'post_type'        => 'post',
    'post_status'      => array('publish', 'draft', 'pending' ),
    'author'           => 1,
); 

$postslist = get_posts( $args );
foreach ( $postslist as $post ) :
  setup_postdata( $post ); ?> 
    <div>
        <?php the_date(); ?>
        <?php the_title(); ?>   
        <?php the_excerpt(); ?>
    </div>
<?php
endforeach; 
wp_reset_postdata();
?>

You can modify it according to your need.

If you want easier solution, you can use plugins. This is the plugin I recommend to you for easier solution: