When to use http status code 400?

the w3c's doentation http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html told me that if the request has "malformed syntax", use 400, but what is malformed syntax? If I am POSTing an empty content to a URI like http://example.com/articles/, which is not allowed, is this request a malformed syntax, can I use 400 bad request in this situation?