htaccess and Non WordPress folders

I am developing a site where the public part is WordPress but there is a private part where members add information about themselves (which eventually also becomes available through the public part).

Let’s say the site is somedomain.com. When the public visit the site they start on the WordPress home page, whatever it is. If there happened to be a page called private, then somedomain.com/private would take them to that page. If it wasn’t there, they get a nice WordPress 404.

I would like to have a folder called private with a login page called index.htm so that members going to somedomain.com/private would be in the right place with a short URL. Unfortunately, under these circumstances, WordPress triggers a server error 500.

I think I need to modify the .htaccess file to sort this out but I don’t know how.

Any advice gratefully received.