I have 5 different forms (5 pages),by filling each form it will go to next form, it will continue till last form.in first form i have a select box, that select box contains two values, called, a and b.
if I click A , I want to hide the third form(from that 5 forms), if i click B i want to show all 5 pages. how to do?

Is this plain HTML or are you developing using server side scripting (php, asp.net, jsp, etc..)? With server side scripting, you can store a value in a session variable which will persist between pages. So, say the user chooses "A", then store something in a variable so that when you get to form #3, you can decide whether to show it or skip it.

It s a server side script. I dont have any user login. Directly values entered by user will be stored in db. So that I dont know how to do

The way I am proposing I have used it in asp.net platform. At the SelectIndexChanged event of your select box (or the analogus if diferent platform) you can check the selected item value against the balue you want. If it is 3 then set the form's visible value to false. If its 5 set all the forms visible value to true.
Hope I helped a bit.

Thank you a lot. It helped me a bit m trying

You really havent provided enough details to get detailed help. For example, what are you developing in? html, php, asp.net, jsp? Where is your sample code (not all of the code, just the relevant) you need help with?

You have been given some pretty good guidance but without additional information about what you are doing, its going to be challenging.