how to use a span tag for form fields in ruby on rails?

When you submit an HTML form, the only values that get POSTed are those that are in input fields such as text fields, selects, checkboxes, buttons, etc. Content that is simply on the page -- in a span or not -- will not get posted back to the server. That isn't a Rails issue, it's just the way HTML works.

I'm not exactly sure what you're trying to do here, but a common approach when you want to display a value (not in an input box) and also post the value back with the form, is to both render the value on the page (in a span or however you want) and also add a hidden input field (hidden_field_tag) that also has the value in it.