How to add dynamic fields with condition? I have a website that is about automobiles, I want when a user choose a brand drupal shows some of its models:

This image show an example:

for example I want when a user selected BMW drupal shows BMW's models in other select tag dynamic. and for example selected Lamborghini  drupal shows Lamborghini's models.

 

I mean a tree stystem
I would recommend you to use any one of following modules.

Hierarchical Select 


  This module defines the "hierarchical_select" form element, which is a greatly enhanced way for letting the user select items in a hierarchy.


Simple hierarchical select 


  Simple hierarchical select defines a new form widget for taxonomy fields to select a term by "browsing" through the vocabularies hierarchy.


Taxonomy Term Reference Tree Widget 


  This module provides an expandable tree widget for the Taxonomy Term Reference field in Drupal 7. This widget is intended to serve as a replacement for Drupal's core Taxonomy Term Reference widget, which is a flat list of radio buttons or checkboxes and not necessarily fit for medium to large taxonomy trees.