In your ergative pairs, the second sentence is equivalent in meaning to a passive sentence - The door was closed, The glass was broken, etc. In your middle construction sentences, there is no exact passive equivalent. There is therefore an underlying difference between the two classes.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.
pructusAfter second thought...On the passives of middle construction....How about, "Butter cuts easily" into "Butter is cut easily", and "This book sells well" into, "This book is sold well"?Aren't these passives that reflect the original sentences?No. If we say "Butter cuts easily", we are not actually saying that it is cut, merely that it can be cut.