Two books, one from the 50s and one from the 90s, treat the English relative pronouns. The history is briefly mentioned on p.132 of Robertson’s & Cassidy’s The Development of Modern English (1934 & 1964):
Old English possessed no distinctive relative pronoun. The relative function was variously performed (1) by the demonstrative se, seo, thaet, (2) by the indeclinable relative particle the, (3) by the joined with the demonstrative, or (4) by the joined with the personal pronoun. The sole remnant of all these forms is that, originally the neuter of the demonstrative, but widely used as the relative pronoun for all genders in Middle English, and the only relative which is in general colloquial use today. The other relatives of Modern English – who and which developed their relative use much later, though both are from Old English forms, the interrogative indefinites hwa and hwilc respectively. Which was the earlier to develop a relative function, it became, in early Middle English use, a general and indeclinable relative form, like that. A familiar instance of its earlier applicaton to persons is the oopening phrase of the Lord’s Prayer, “Our Father, which art in Heaven.” The present use of who as a relative and the distinction between who for persons and which for animals and for things were not fully worked out until the eighteenth century. {note}. Even yet, as has been suggested, who and which are used as relatives less freeely than as interrogatives; the man in the street unconsciously prefers that to who – partly, no doubt, because by using that he avoids the uncomfortable choice between who and whom.
{note} The classical illustration is “The Humble Petition of Who and Which,” in the Spectator for May 30, 1711. As Jespersen points out (Essentials of English Grammar, p 359, Addison turns historical truth topsy-turvy by describing the relative that as an upstart that hs recently done injury to who and which.
Just a quick comment on the note. Addison is a revered figure in English literature and Jespersen is a revered figure in the study of English linguistics; who do you think English teachers will trust more?
To current times: Celce-Murcia and Larsen-Freeman in their The Grammar Book list several modern English relative pronouns with notes on their use for ESL teachers to base their lessons on. Page 371 states:
who + subject case
+ human
whom + object case
+ human
Although it would always be prescriptively correct to use whom to replace a human NP in object position – or as the object of a preposition without a fronted preposition – native speakers do not always use it this way. Often they opt for the subject case form, who, instead:
I spoke with the student who(m) I loaned the book to.
On the other hand, if the who(m) is replacing the object of the preposition and the preposition is fronted with it, whom will almost always be used:
I know the student to {whom} you loaned the book.
{*who}
which – human
that + human
–
[the + means used with either human or non-human.]
–
In informal conversational discourse, that is preferred over either which or who(m). In writeen discourse, which and who(m) are preferred; when that is used it usually indicates a nonhuman head noun (Stauble, 1978)
The Stauble citation refers to Stauble, A. (1978). “A Frequency Study of Resrtictive Relative Clause Types and Relative Pronoun Usage in English”. Unpublished English 215 paper, UCLA, fall, 1978.
What can we say about this in reference to the post that states:
“The thing I find most upsetting is that none of the other “professors” could
speak English either, so I must have seemed an oddball. No one knew the
difference between ’which’ and ’that’ or ’that’ and ’who,’ for example. I
realize that linguistic drift is ridding us of these pesky relative
pronouns, but, of course they’re still alive and well in French, so they
sound awful to me when they’re wrong in English.”
Was she referring to formal written works or to formal discourse or was she referring to a classroom lecture style or friendly conversation among peers (little did they know)? What we are told is that they don’t speak English. How linguistic drift is getting rid of relative pronouns is impossible to contemplate for those of us who do speak English. Perhaps the writer was referring to the deletion of the relative pronoun when it is the object of its clause, but that usage has been around since Old English and is part of formal discourse.
I think it’s time to be honest here. I cannot take a year or so out of my life to determine exactly what this teacher meant and just what her circumstances are; I certainly cannot repeat the instances where she heard her professor colleagues’ bad grammar. All I can do is place her post in a context with many like it on listservs inhabited by language teachers and what I get when I do that is a very snide person who sets herself up as superior to others based on grammar shibboleths she supposedly observes and others are too stupid to observe or even know. We see this time and again.
Right now I am reading a book written in 1965 by Gleason, a foremost structural linguist of the time, wherein he outlines the history of grammar teaching in U.S. schools, the origins of these grammar shibboleths, and what we can do to teach grammar in the schools. One thing I’ve already read that really hit home with me because it recalls what has happened to me on some listservs: if you question anything about the received grammar as taught by fifth grade teachers the country over, you are abandoning grammar altogether and are totally permissive regarding usage, condemning students to lives of disgrace due to their poor English. Oh yes, it gets worse: as more and more people fail to learn and apply the rules (there are about eleven of them that teachers teach), our entire society will fall into dumb decay and chaos.