How to generate subform of form - Zend Framework
<?php
   $username = new Zend_Form_Element_Text('user');
   ...//param for field
   $password = new Zend_Form_Element_Password('pwd');
   ...//param for field
   $name = new Zend_Form_Element_Text('name');
   ...//param for field
   $submit = new Zend_Form_Element_Submit('submit');
   ...//param for field
   $this->addElement(array($user,$password,$name,$submit));
   $this->addDisplayGroup(array($user,$password,$submit),'login');
   $this->addDisplayGroup(array($user,$password,$name, $submit),'create');
?>

class LoginForm extends Zend_Form
{
    public function init ()
    {
        $this->addElement('text', 'user');
        $this->addElement('password', 'pwd');
        $this->addElement('submit', 'submit');
    }
}

class RegisterForm extends Zend_Form
{
    public function init ()
    {
        $this->addElement('text', 'user');
        $this->addElement('password', 'pwd');
        $this->addElement('text', 'name');
        $this->addElement('submit', 'submit');
    }
}

class BaseForm extends Zend_Form_Subform
{
    public function init ()
    {
        $this->addElement('text', 'user');
        $this->addElement('password', 'pwd');
    }
}

class LoginForm extends Zend_Form
{
    public function init ()
    {
        $subform = new BaseForm;
        $this->addSubform($subform, 'base');

        $this->addElement('submit', 'submit');
    }
}

class RegisterForm extends Zend_Form
{
    public function init ()
    {
        $subform = new BaseForm;
        $this->addSubform($subform, 'base');
        $this->addElement('text', 'name');

        $this->addElement('submit', 'submit');
    }
}

public function loginAction ()
{
    $form = new LoginForm();
    // More code here

    $this->view->form = $form;
}

public function registerAction ()
{
    $form = new RegisterForm();
    // More code here

    $this->view->form = $form;
}

$mySubForm->addDecorator('HtmlTag', array('tag' => 'dl', 'class' => 'zend_form'))
          ->addDecorator('Form');

<?php echo $this->mySubForm; ?>