4. Hence the relative is the equivalent to a demonstrative pronoun, or to a substantive, indifferently.
5. But the relative is the equivalent to the pronoun and substantive, and something more. In sentences like
The man is come—he rides—
The man is come—the man rides.
The identity between the person mentioned in the two propositions is implied, not expressed. This the relative expresses; and hence its use in languages.
6. From these observations we get a practical rule for determining doubtful constructions.
a. Reduce the sentence to the several propositions (which are never less than two) which it contains.
b. Replace the relative by its equivalent personal or demonstrative pronoun, or by its equivalent substantive.
c. The case of the demonstrative or substantive, is the case of the relative also.
By applying this rule to such expressions as