The user should see a 'New Reply' link or button on a post's show page which takes them to a form. The form should include the title of the post being replied to.
The user should be able to create a new reply to a post using the 'New Reply' form.
The user should see all the replies to a post on the post’s show page.
If the user doesn't submit all required fields, they should see some error messaging, but they shouldn't lose any of their work.
What is a nested resource? When is it appropriate and how does it help?
How do the RepliesController and PostsController interact?
What should happen to the routes file for this nesting business to work?
RailsGuides - Rails Routing from the Outside In http://guides.rubyonrails.org/routing.html
Go on to Inline Replying On A Post