The functionality exposed by forums to members is controlled via permissions and configuration options. The default permissions for forums within a group can be managed by navigating to the group, Manage > Manage Group > Permissions and reviewing the forum permissions granted to a specific role. To manage the configuration of a forum, go to the forum, Manage > Manage Forum and review the options there. To manage permissions for a specific forum, navigate to the forum, Manage > Manage Forum > Permissions.
For more details on managing permissions, see How do I manage roles and permissions to control what members can do on the community?