how to dislplay previous post() content

If you want a meaningful answer – give more details. Just describe what you want to do, what is your goal, and let us tell you which template tag to use 🙂

1. There is no such tag that you put in the title here.

2. Since you seem to be in love with that site – why not ask the author?

3. Probably a recent_posts plugin will do it – search the Codex: http://codex.wordpress.org/Plugins