How to comment on a tweet on the iPhone app?

Okay, so say someone you follow has tweeted, I want to know how people's comments are coming up below the tweet and mine seem to just post to their profile? There's no obvious reply butting on the app. Every time I've tried to post a comment on the tweet it just doesn't go where all the other people have commented its goes on their profile?

Twitter refreshes iPhone app interface, adds new Tweet composer, notifications when friends sign up

Twitter has updated its application for the iPhone and iPod touch with a refreshed user-interface for the Tweets timeline.

Additionally, the updated application features a new Tweet composer interface that actually allows users to preview what a posted Tweet will look like in the timeline. Also notable is the new ability to receive notifications when friends join Twitter.