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You can often subscribe or unsubscribe to an individual forum. Note that the instructor may "force" or "disable" subscriptions or may leave them at the default (its your choice settingbased on your profile preferences as shown above).Click on the link to a forum.
Look left to the Administration block and you should see "Forum Administration". Just click the Subscribe or Unsubscribe option to set your preference for that forum. If your course uses the Boost theme, then there will be a subscribe/unsubscribe link at the top right of a forum when you view a post.
or 
On the main course page look for the Activities block on the right, and click on Forums.
Then in the Subscribed column click on the Yes or No to change your subscription setting for each forum.
