How to add excerpt to posts ?

WP automatically recalls the first 50 words of your Blog Post and displays them on Post Previews, including the html code. For this reason, if you open an italics tag for the text, it will not see the closing tag. To avoid this, and to get a shorter text preview, we suggest using the “Excerpt” field control when you are in the Edit page of your Blog Post.

View more info to codex.wordpress.org/Excerpt