Help - I'm doing something wrong!

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I just joined this morning. I'm able to post new discussions and I've been able to respond to some but not all of the comments. I also haven't been able to respond to someone elses discussions. I click on add comment, write a message and click preview - most of the time my message disappears and I'm taken back to a list of articles. I'll keep trying. Does anyone know what I'm doing wrong?