Ergativity and middle construction

Hello, dear Gurus and Members!!

I have been having some hard time trying to understand the difference between ergatives and middle constructions.
With the help of fivejedjon, I feel I found some clue.
So I wrote something about it, to be confirmed by the Gurus here.
There seemed to be nowhere to ask about this part, so I wrote myself to be reviewed and commented here.

This is what came to my mind after some struggling, and I know that native speakers are not much aware of this part in English grammar.
But I'd like to know what you Gurus think about this.
I am a struggling ESL student, so downright frank comments would be most expected.
I hope this is close to what you natives already have in your sense of English.

ergativity and middle construction

There are two concepts related to passive voice. One is ergatives and the other is middle constructions.
Let’s take a look on the ergative pairs below.

Transitive	Passive	Ergative
Peter closed the door.	The door was closed.	The door closed.
I broke the glass.	The glass was broken.	The glass broke.
The wind shook the trees.	The trees were shaken in the wind.	The trees shook in the wind.
I roasted the meat in a hot oven.	The meat was roasted in a hot oven.	The meat roasted in a hot oven.
He flew a plane overhead.	The plane was flown overhead.	The plane flew overhead.
He crashed his car into a tree.	His car was crashed into a tree.	His car crashed into a tree.
He melted the chocolate.	The chocolate was melted.	The chocolate melted.

The three column pairs is called ergative pairs and ergative verbs are those that can show that kind of pairs. We can say ergative verbs and ergative sentences.

Another is middle construction or sometimes called middle sentence. Examples are in the table below.
This constructions are syntactically intransitive but semantically passive.
The middle constructions, unlike ergatives, don’t show the structure of “Transitive—Passive— Middle construction”
If we try to show it, it will look like below.

	Transitive	Passive	Middle construction
1	People cut butter easily.	Butter is cut easily by people.	Butter cuts easily.
2	They sell this book well.	This book is sold well.	This book sells well.
3	People iron the clothes well.	The clothes is ironed well.	The clothes iron well.
4	People wash some paint well.	Some paint is washed well.	Some paint washes well.
5	People don’t frighten her easily.	She isn’t frightened easily.	She doesn't frighten easily.
6	They install the program in seconds.	The program is installed in seconds.	The program installs in seconds.
7	They clean this oven easily and effectively.	This oven is cleaned easily and effectively.	This oven cleans easily and effectively.

In the first row, “Butter cuts easily” and “People cut butter easily” are not of the same meaning. “Butter cuts easily” means that the material butter has the general property of being easily cut. But “People cut butter easily” means that no matter how hard the material butter is, people are cutting butter easily.
And “Butter is cut easily by people” also means “Butter is easily cut already", which is not the same meaning with “Butter cuts easily”.
Therefore, the middle constructions don’t show the structure of “Transitive—Passive— Middle construction”.

Sometimes we see scholars disagree whether a given sentence is ergative or middle construction.
You-Mee Sohn says that the following two sentences are considered as ergatives by Fagan, but she disagrees. (Interpretation of English Middle Constructions, footnote number 5).

Glass breaks easily. // Bombs explode easily.

If we consider the two sentences as ergatives, “Glass breaks easily” means“Glass is being broken easily”, and “Bombs explode easily” means “Bombs are being exploded easily”.

If we consider as middle constructions “Glass breaks easily” means “Glass has the property of being easily broken” and “Boms explode easily” means “Bombs has the property of being easily exploded”.