I want to make a semantic bulletin board.
But, I don't know whether the <section> tag should be used for a category and a forum or not.. And, I also wonder if there is something better if I use <article> tag for the first forum post (the article that is discussed about)

Should <section> tag be used for a category and a forum ? Should <article> tag be used for the first forum post ?

Please share your ideas..

According to the w3schools <section> is "The <section> tag defines sections in a document, such as chapters, headers, footers, or any other sections of the document." means that a section is a separate part of your page that can contain its own h1 or other crucial tags.
The <article> tag on the other hand is defined as "The <article> tag specifies independent, self-contained content.

An article should make sense on its own and it should be possible to distribute it independently from the rest of the site.

Potential sources for the <article> element:

Forum post

After all these I would recomend using <article> for your forum posts but keep <section> for a "bigger" part of your page except if you constract your categories pages to much the <section> definition!