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Only administrators and moderators can perform these actions. Closed topics will be invisible to regular users. Moderator can only close or open topics in the category that he manages.
Select a topic by ticking its relevant checkbox, then click on the Action bar if you are in a topics list;
Or, click on the Action bar if you are in a topic.
Click Close or Open from the drop-down menu.
You can also close/open a topic while editing the topic by ticking/unticking the Close the topic checkbox in the Options tab.