How to a short form of posts on main page

consider to use the_excerpt() to output the short summaries on the main page;

http://codex.wordpress.org/Function_Reference/the_excerpt

for details, please contact the theme’s developer via themeforest for support.

commercial themes are not supported by this forum.

There’s also a ‘more …’ option you can include when writing the post, which breaks the content at that point on the home page, but shows the whole post content on the post page.
(I tend to leave it out when first writing a post unless its really long, and add it later when the post shuffles down the list.)