A Practical and Authoritative Guide to Contemporary English. 1996.
Page 50
the preposition to, and The man whom the papers criticized did not show up, since whom is the object of the verb criticized.
1
This all seems straightforward enough, but with complicated sentences it is not so easy. A sentence such as I met the man whom the government had tried to get France to extradite requires you to have thought the sentence through before you have written it—you should know from the start that whom will be the object of the verb extradite, which is several clauses away. It is hard to be this calculating on a consistent basis, so it’s not surprising that writers from Shakespeare onward often use who and whom interchangeably. In speech and informal writing, people tend to use who, even as the object of a verb or preposition. A sentence such as Who did John say he was going to support? is perfectly natural, despite violating the traditional rules. Using whom often sounds forced or pretentiously correct, as in Whom shall I say is calling? or Whom did you give it to? Nevertheless, many writers adhere to the rules, especially in formal style. These rules apply in the same manner to whoever and whomever.
2
who in restrictive and nonrestrictive clauses
The relative pronoun who may be used in restrictive clauses, in which case it is not preceded by a comma, or in nonrestrictive clauses, in which case a comma is required. Thus you can say either The scientist who discovers a cure for cancer will be immortalized, where the clause who discovers a cure for cancer indicates which scientist will be immortalized, or The mathematician over there, who solved the four-color theorem, is widely known, where the clause who solved the four-color theorem adds information about a person already identified by the phrase the mathematician over there.
