The Classification of Words

§ 160. This — these, that — those are number opposemes. The grammatical meanings of 'singular' and 'plural number are also preserved when these pronouns are used as pro-adjectives, thus involving grammatical combinability: this book, these books.
As elsewhere the 'singular' members of the opposemes occur in speech much more often than the 'plural' opposites, the ratio being approximately 9:1.
Interrogative Pronouns
§ 161. The meaning that unites the interrogative pronouns is "an inquiry" about some object (what, who), its properties (whose, which, what), place of some event (where), its time (when), cause (why), manner of existence (how).
107
As seen from the above, this group embraces pro-nouns (wfut, who, which, whose), pro-adjectives (what, which) and pro-adverbs (where, when, why, how).
Who and what are regularly used to distinguish "persons" and "non-persons". What is, ho\ve\er, used in reference to living beings as well when the inquiry concerns the occupa­tion of the person spoken about as in What is he? He is a student.
§ 162. The pronoun who is the only interrogative pronoun which has a case opposite, whom, as in Whom did you meet?
However we observe here a distinct tendency to substi­tute the nominative case (who) for the objective (whom), which may eventually bring about the obliteration of case distinctions in the interrogative pronouns.
E. g. I went to the pictures. Who with? ' Who are you trying to deceive? (Buck)
Note. The unmarked member of the case opposeme who — whom occurs, as usual, more often than the marked member (whom). The ratio is approximately 9:1.
§ 163. The interrogative pronouns are used to form a definite type of sentence — special questions.
W h о told you tliat? What is the charge? (Hanley).
Charles Fries writes: "The expression Who came? signals a question not because of a different arrangement, but solely because the signal of question is in the word who as a word" ².
Connective Pronouns.