problem when adding jquery.js file I need to run jquery code in my content type form so i added jquery-1.8.0.min.js file in info file of my theme.  

after clear all caches something wrong happened, the "add more" button on date field stop working, i'm sure it's from adding the js file.  

Any suggestion?
Drupal already includes jQuery. You should not add another jQuery library because this can cause conflicts with the existing one.

I suggest you to use jQuery Update if you want to use another version of jQuery.

If what you want is to run your own javascript code, you could add it using drupal_add_js().

Hint: Try first with alert('hello world'). See Working with JavaScript and jQuery.