Over the past five years, my high school students have stopped using "through" as a preposition and use "by" almost exclusively. For example, they might write, "they will do this by a detailed research project". I have looked up the answer and I can't figure out a way to explain the difference to them, and I almost think that I just need to let it go. Does anyone know why these two words have become virtually interchangeable?
Prepositions are perhaps the most polysemous of words. And all words are polysemous.
It's tempting to say 'You just have to learn every individual collocation'. So don't start teaching until you're past retirement age (by which time usages will have changed anyway, so you'll have to go on a refresher course ...).
As jimsug says, there are instances where the two prepositions are definitely not interchangeable; often, they are highly idiomatic:
I learnt it by heart/rote.
*I learnt it through heart/rote.
*She stuck with him by thick and thin.
She stuck with him through thick and thin.
Though they may be central senses (directional / locative):
I drove by the site of the old Chicago Motor Speedway. =/=
I drove through the site of the old Chicago Motor Speedway.
In 'they will do this by / through a detailed research project', the more peripheral sense/s of agency/means are in play:
I could tell it was you by what you were wearing.
He made it by hand.
We went by car.
He only discovered this by chance / through good fortune [far more common than the next alternative ?!] / by good fortune.
You will only succeed through hard work / by working hard.
'They will do this by a detailed research project' is possibly best considered as an ellipted form of
'They will do this by carrying out a detailed research project' or
'They will do this by means of a detailed research project' or
'They will do this through the expediency of carrying out a detailed research project' or
'They will do this through the expediency of a detailed research project'.
This is perhaps an illustration of part of the evolutionary processes that can be involved.
In this case, looking to the etymology of the words helps. According to the OED, both by and through can be traced back to Old English (though, through does go back beyond that). The key difference here is that through is actually derived from thorough, and as such refers to the sole means through which something is accomplished, from start to finish. By was initially in contrast to byway, where a byway was the primary path of travel and a by was the secondary.
Essentially, through trumps by in terms of importance of the object of the preposition, and in prescriptive rules they cannot actually be used interchangeably. Descriptive English, however, allows them to be used interchangeably, because people already do.
Looking at some definitions from Collins:
I'd say because of the overlap in meaning and usage (through is even used as a definition for "by"), people (and not just kids, although they seem to have quite the role in language change) use them interchangeably.
You should be able to demonstrate the differences by providing examples where they're not interchangeable, eg:
He was accepted as a member of the bandicoot totem by his own male relatives
*He was accepted as a member of the bandicoot totem through his own male relatives
It's possible to improve your fitness through exercises.
*It's possible to improve your fitness by exercises.
I would probably let your example slide, as it communicates a similar process/event, and is grammatical with either by or through.
A --> B --> C
From a through (tunnel b) to c.
From a via (road b) to c. (via is way or road in latin)
From a by car via b to c.
From a with car through b to c.
From a by car by b to c. Nope. Passing by maybe.
By --> by way of (over, passing by, but not passing through) through --> you need to you through the wood in order to come out on the other and there is no way to pass it by.
So through is like a roadblock that you need to take in order to arrive on the other end of the road. Aka. a mountain with a tunnel.
If one learns something "by means of" X, it is neither "through" nor "by," and indicates that a thing was learned by means of a certain process, whether that be education, experience, scientific inquiry, etc.
Question. Examples given: He was accepted as a member of the bandicoot totem by his own male relatives, and, *He was accepted as a member of the bandicoot totem through his own male relatives. If the asterisk is used to indicate correct usage, that is wrong. "By" is correct. He did not travel "through" his relatives.