Use course tools>>Discussion Board>>Participate in a forum
You can mark selected messages as read or unread in a Discussion Board forum. The titles of unread messages appear in bold.
Click Show Options on the far right above the messages or click to display the option buttons.
Do one of the following:
Select each message you want to mark as read or unread by clicking the box to the left of it so a checkmark appears .
Select all messages by clicking Select All.
Click either Read or Unread.