Beginner - default form values

I want to build a simple calculator with 1 form and a results page.
The form will be redisplayed with an error message upon an invalid value
I won’t be using any database/active record for this.

The problem i have is when i return to the form page i want to redisplay
the values already entered in the form fields.
Should i use text_field (which seems to be tied to a model) or
text_field_tag and how do i pass these values submitted back to the form
again from my controller.

any help would be appreciated.
cheers, simon.