Complex sentence: The yawl is often confused with a ketch but the ketch has the mizzen mast forward of the rudder post whereas the mizzen on a yawl is aft of the rudder post.
Split into simpler sentences: The yawl is often confused with a ketch. The official difference is that a ketch has the mizzen mast forward of the rudder post whereas the mizzen on a yawl is aft of the rudder post.