implementing struts action classes

Guys, I am trying to implement Struts in my java app and replacing my servlet controllers using Struts action classes. I have 2 questions related to it.

1. Do I need to replace all my servlet controllers with Struts action classes? If not, how do i identify which one to be replaced?

2. Can I forward the action class to another servlet instead of a jsp page?
Or does it defeat the purpose of using struts?

3. Do I need to create an Action and a Form class everytime? Can my newly created Action class use a previously created Form class?
For example, say I have LoginForm, LoginAction and AdminAction. Can AdminAction access LoginForm? or does it has to be AdminForm itself? because the LoginForm & AdminForm uses the same code.