[Zope] login page problem

I'm creatin a customised login page template using districtname, user name and password. i got halt in d logic for password checking. after submitting d login form how do i call d same page if the passwd is wrong along with d req params passed? do i need an ext python script for this redirection. also tell how to set session data in ZPTs from the form fields? -- Share the vision of difference with ME