I want to make a mandatory checkbox which says "I understand the terms and conditions of ... "


Use these settings.  Don't forget to set the width of the field wide enough to accommodate the text.

Enter only a single key=value pair in the Default Value configuration field.

In this example, a value of "understand_terms" will be returned if the user checks the checkbox.

If the user fails to check the checkbox, the form will *not* be submitted.  If you want the form to be submitted even if they do not check this checkbox, change the option next to Activation from Requiredto Yes.