WordPress Permalink

This might help you:

Go to your Settings => Permalinks => Cutom Structure

And paste this:

/category/%category%/article/%postname%/

Also paste this on Category Base field:

category

This will work for category but when you visit any post you will be welcomed with 404 page not found (Isn’t it nice?).

So stealing from this answer

Add this code to your themes functions.php file:

add_action( 'init', 'wpa58471_category_base' );
function wpa58471_category_base() {
    // Remember to flush the rules once manually after you added this code!
    add_rewrite_rule(
        // The regex to match the incoming URL
        'category/([^/]+)/([^/]+)/([^/]+)(/[0-9]+)?/?$',
        // The resulting internal URL
        'index.php?category_name=$matches[1]/$matches[2]&name=$matches[3]&paged=$matches[4]',
        // Add the rule to the top of the rewrite list
        'top' );
}

And switch back to Settings => Permalinks and click Save Changes.