You can change it either in your User CP under Options, or change it in the thread under 'display modes' in the top-right corner of the thread.
however, I'm guessing that 99% of the people use and are used to the default linear mode.
If you want to reply to a specific person, just use the 'quote' button
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- + - ° GiRLie GeeK ° - + - °
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Therell be days/When Ill stray/I may appear to be/Constantly out of reach/I give in to sin/Because I like to practise what I preach