In some wordpress themes that you can find for free, you get full posts on main index page. In some cases you have optins on theme settings to get excerpt and not full post, but some of them doesn’t have that option. So here is how to fix it if you really like that theme and you don’t wanna change it. You have to go in your theme editor and open index.php and search for the_content () or in some themes you have to search of included page like post.php or similar. If you can find the_content (), you just need to change it to the_excerpt() and you’l get “short” posts or preview.
Than your main page will show posts like this: