Prepositions in Relative Clauses: Placement Rules and Exceptions (during which)

Question

"In the form of 'preposition + relative pronoun (whom, which),' we can place the preposition at the end of the phrase. For example:

1. I read a book in which I was interested. (✔)
2. I read a book which I was interested in. (✔)

However, I've observed that in the form of 'during + relative pronoun,' we cannot place 'during' at the end of the phrase. For instance:

1. The 50 days during which we tried so hard were not useless. (✔)
2. *The 50 days which we tried so hard during were not useless.

Now, I have two questions:

First, why can't we place 'during' at the end of the phrase? Second, are there any other prepositions that cannot be placed at the end of the phrase?

I would appreciate your assistance. Thank you."

Answer 1

Some answer to these questions is found in A Comprehensive Grammar of the English Language, 1985 edition. (user LPH's bold type in the text below; the full text extracted from CoGEL has been shortened to its essential, but with an eye to the preservation of what seems to put the question into perspective.)

(CoGEL § 9.6) Deferred prepositions
[…]

With interrogative and relative pronouns as prepositional complement, there are often alternative positions available: one formal with the preposition in its normal place before the complement […], the other informal with the preposition deferred to final position […]:
WH-QUESTIONS […]

RELATIVE CLAUSES […] :

• The old house about which I was telling you is empty. (formal) [2]
• The old house (which) I was telling you about is empty. [2a]

A prejudice against such deferred (or 'stranded') prepositions [2a] remains in formal English which, […] for relative clauses, offers the alternative of an initial preposition [2]. The alternative construction is often felt, however, to be stilted and awkward, especially in speech. In some cases, such as the following, the deferred preposition has no preposed alternative:

• What did, she look like?

• What I'm convinced of is that the world's population will grow to an unforeseen extent.

• All she could talk about was her dog.

In general, it is the most common and the short prepositions which can be deferred, in particular spatial prepositions […], compare:

• He left his coat in the car.
  ~¹. . . the car in which he left his coat
  ~. . . the car (that) he left his coat in

• He left politics because of the election results.
  ~. . . the election results because of which he left politics
  ~. . . the election results *²(that) he left politics because of

• The plane was destroyed through the pilot's carelessness.
  ~. . . the pilot's carelessness through which the plane was destroyed
  ~. . . the pilot's carelessness *(that) the plane was destroyed through

¹ ~: _{systematic correspondence between structures}
² *: _unacceptable

More precise information can be had from CoGEL.

(CoGEL § 17.17) Relative pronoun as adverbial

When the relative pronoun is the complement of a preposition (and, together with the preposition functioning as adverbial), some choice exists in placing a preposition which has a wh-pronoun as its complement. No such choice exists with that, where postposition with deferred preposition represents the sole pattern:

• the lady towards whom the dog ran
  the lady who(m) the dog ran towards
  the lady that the dog ran towards
  the lady () the dog ran towards

• the table under which the boy crawled
  the table which the boy crawled under
  the table that the boy crawled under
  the table () the boy crawled under

In general, it is certainly true that wh-pronouns with initial preposition are used predominantly in formal English: […]
Initial prepositions are normally avoided in more informal use, where they would be felt to be stilted or pompous.
[…]
[…] many prepositions (especially those dealing with temporal and other abstract relations) cannot easily be deferred […]:

• ?That was the meeting (that) I kept falling asleep during.

One might find [3a], but in familiar speech an adverbial relative with when or where would be preferred to during which:

• That was the meeting during which I kept falling asleep. [3a]

Prepositions expressing spatial relations allow a deferred preposition even when the preposition is complex:

• This is the house he stood in front of.

³ ?: _{native speaker unsure about acceptability}

The ultimate answer to "why can't we" is really a matter of psycholinguistics (Wikipedia: Psycholinguistics is concerned with the cognitive faculties and processes that are necessary to produce the grammatical constructions of language. It is also concerned with the perception of these constructions by a listener.); why some of the possibilities examined above are not felt appropriate is to be sought in the impressions and taste of the native speakers subsumed as a collective conclusion, something that in grammar and linguistics yields what is called technically usage, usage being, roughly speaking, the fact that people say things one way rather than another. What has been said above (CoGEL) aims at determining mere usage, but as far as a why is concerned all that can be said are things such as "felt to be awkward… stilted… pompous"; it remains to know the why of those impressions (much research of which I don't even know the most elementary facts, but that research does not concern grammar).