If your WordPress-powered site includes a blog as a component, rather than the main function, you can opt to put blog posts and archives (category/tag/date/author) into their own subdirectory (such as “blog”).

Here’s how to do it:

First, login to your WordPress administration. Click “Settings” (or “Options”), then click “Permalinks.”

After the beginning slash in the “Custom Structure” box, add the subdirectory name followed by another slash.

For example, if your permalink structure was this:

/%year%/%monthnum%/%day%/%postname%/

…You would change it to this:

/blog/%year%/%monthnum%/%day%/%postname%/

Click the “Save Changes” button. Now your post and archive permalinks will contain the subdirectory name that you entered.