Here are some Drupal AHAH Form examples using the Drupal 6 Form API and the wonderful AHAH Helper module. It's much easier creating AHAH forms with the AHAH Helper module than without using it.

Install example_forms module and ahah_helper module and then you can look at:

1. [yourdomain]/example1
2. [yourdomain]/example2
3. [yourdomain]/example3
4. [yourdomain]/example4
5. [yourdomain]/example5
6. [yourdomain]/example6
7. [yourdomain]/example7

and take a look at the example form code in example_forms.module to see how to create Drupal AHAH forms.

Have fun!