(4) It has the same form for singular and plural.
It sometimes borrows the possessive _whose_, as in sentence 6, Sec. 109, but this is not sanctioned as good usage.
117. The sentences of Sec. 110 show that--
(1) _What_ always refers to things; is always neuter.
(2) It is used almost entirely in the singular. 1. The man _that_ hath no music in himself,... Is fit for treasons, stratagems, and spoils. --SHAKESPEARE (3) Its antecedent is hardly ever expressed. When expressed, it usually follows, and is emphatic; as, for example,--
What I would, _that_ do I not; but what I hate, _that_ do I.--_Bible_
What fates impose, _that_ men must needs abide.--SHAKESPEARE.
What a man does, _that_ he has.--EMERSON.
Alas! is _it_ not too true, what we said?--CARLYLE.
118. These are the forms of the simple relatives:--
_Nom._ who which that what _Poss._ whose whose -- -- _Obj._ whom which that what
119. The _gender_, _number_, and _person_ of the relatives _who_, _which_, and _that_ must be determined by those of the antecedent; the _case_ depends upon the function of the relative in its own clause.
For example, consider the following sentence:
"He uttered truths _that_ wrought upon and molded the lives of those _who_ heard him."
Since the relatives hold the sentence together, we can, by taking them out, let the sentence fall apart into three divisions: (1) "He uttered truths;" (2) "The truths wrought upon and molded the lives of the people;" (3) "These people heard him."
_That_ evidently refers to _truths_, consequently is neuter, third person, plural number. _Who_ plainly stands for _those_ or _the people_, either of which would be neuter, third person, plural number. Here the relative agrees with its antecedent.
We cannot say the relative agrees with its antecedent in _case_. _Truths_ in sentence (2), above, is subject of _wrought upon and molded_; in (1), it is object of _uttered_. In (2), _people_ is the object of the preposition _of_; in (3), it is subject of the verb _heard_. Now, _that_ takes the case of _the truths_ in (2), not of _truths_ which is expressed in the sentence: consequently _that_ is in the nominative case. In the same way _who_, standing for _the people_ understood, subject of _heard_, is in the nominative case.