Some themes let users show post excerpts in the home and other pages by default. But you will be insisted to show full posts on home, category and archive pages of rest of the themes. You can’t change their inbuilt settings, even though you opted for post summary in the settings panel (In your WordPress dashboard, Settings -> Reading -> Post Summary). Make sure that you are activated Post Summary option, before going to the rest of the topic.
If it is not working, go to Appearance -> Editor -> Home.php or index.php of your theme.
Find the following line from the codes,
<?php the_content(); ?>
replace it with
<?php the_excerpt(); ?>
Save the changes.
Clear your caches and check your home page, it should display post excerpts rather than full posts now. If still not working, try to use the following tag in your post at the point where you want the post’s content to stop displaying on the front page.
But the main drawback of this option is that you need to insert this tag in every post.
There’re some plugins doing this work more effectively. Easy Custom Auto Excerpt is such a plugin. It lets you set excerpt size on each page and customize other options like images, read more button or text, alignment etc.
You can go for Advanced Excerpt also. It is useful to show excerpts in terms of a number of words and sentences. You can activate or deactivate the feature on home, category, archive, search and author pages.