I just isntalled geeklog and want to install the forum

can you please tell me which plugin to install for Geeklog 1.4.0

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what is: white black white black white black white black
answer: a nun falling down the stairs

The name of the forum plugin is 'forum plugin'. if you use that as a search string up there on the left hand side search bar, then you will get a page of results matching your criterion. look around, and you should be able to download the forum plugin.
If not, try Squatty's website or try Blaine's website for your queries.
I would suggest, before you ask for help at the forums, to search for what you want on these three websites.