How to modify comments form using comment_form()?

how i can modify comments form, my theme just call:

<?php comment_form(); ?> 

Do you mean modifying its text or behavior? The function is very extensive and customizable. It both accepts extensive set of parameters and has a lot of hooks.

For starters see:

• comment_form() in Codex;
• WordPress 3.0 Theme Tip: The Comment Form.