Google Docs templates, there are several business forms ready to choose from.
Use the following general form template to get you started quickly.
Click 'Use template' and the template will be saved automatically.
In your Google docs account you now have a copy - 'Copy of Contact us' form.
In your Google Site: Goto
Insert > Spreadsheet Form > Select 'Copy of Contact us' form.
Date Calculations Form
You can also choose from several Design Templates to make your form more styled to suit your Google Site.
If you manually style the form in HTML it will revert back to the original with multipages or any submission errors.
When Google Sites Form is Submitted:
Develop Advanced forms with Google's' Apps Script
To Modify this form just go back to all your Google docs and you can modify sharing options etc... Google Docs
Click on the Form file to modify the fields and other options like sharing/confirmation etc...