_That_, _what_, _whichever_, _whichsoever_, _whatever_, and _whatsoever_ are not declined. They have the same form in the nominative and objective cases, and are not used in the possessive case.
_What_ is peculiar in that it never has an antecedent expressed, but itself stands for both antecedent and relative. It is called the _double relative_. Compare the following:
_That_ is called the restrictive relative, because it limits or restricts its antecedent to the meaning expressed in the clause introduced by _that_. A restrictive clause is one, therefore, that is needed to make the meaning of the sentence clear. Compare the following:
_Non-restrictive_: John Brown, _who_ has no disease, needs no physician.
_Restrictive_: He _that_ hath no disease needs no physician.
Notice that a restrictive, or necessary, clause is not separated from the rest of the sentence by commas.
_Who_ and _which_ are sometimes used with restrictive force; as,
1. Those _who_ have finished their work may leave. (Not everybody.)
2. Have you read the book _which_ he recommended? (He recommended but one.)
_Interrogative_ pronouns are used in asking questions. They are _who_, _which_, _what_. _Who_ refers to persons; _which_ refers to persons or things, and is used to distinguish one object from another; _what_ refers to things. They are declined as follows:
_Nom._ who which what _Poss._ whose (of which) (of what) _Obj._ whom which what
The interrogative pronouns _which_ and _what_ are frequently used as adjectives. In this case they are called _pronominal adjectives_. Compare:
Pronoun: _Which_ of these hats do you prefer?
The _demonstrative_ pronouns are _this_ and _that_ with their plurals _these_ and _those_. They are always used to point out, or demonstrate, the noun to which they refer. _This_ and _these_ are used for objects near at hand, or recently named; _that_ and _those_ are used for objects far away, or not recently named.
The demonstrative pronouns are frequently used as adjectives; as,
Pronoun: _That_ is my book. Adjective: _That_ book is mine.
_Indefinite_ pronouns refer to objects or persons, but do not define or limit them. The indefinite pronouns are _each_, _every_, _either_, _neither_, _one_, _none_, _other_, _another_, _few_, _all_, _many_, _several_, _some_, _each other_, _one another_, and the compounds _any one_, _some one_, _every one_, _something_, _nothing_. Indefinite pronouns are frequently used as adjectives. _Each_, _every_, _either_, _one_, _another_, _any one_, _some one_, _every one_, whether they are used as pronouns or as adjectives, are singular in number. If another pronoun is used to refer to one of them, it must be in the singular number.