Seems like all you need to do to get @comment form working under each @tweet on index page is:
- @tweets.each do |tweet|
= tweet.name `#to show tweet`
- tweet.comments.each do |comment| `#to show each tweet comment`
= comment.content
= form_for [tweet, Comment.new] do |f| `#form forcomment belonging to this particular tweet`
= f.text_field :content
= f.submit
Hope it helps :)