Parsing what, the simple relative.
120. The relative what is handled differently, because it has usually no antecedent, but is singular, neuter, third person. Its case is determined exactly as that of other relatives. In the sentence, "What can't be cured must be endured," the verb must be endured is the predicate of something. What must be endured? Answer, What can't be cured. The whole expression is its subject. The word what, however, is subject of the verb can't be cured, and hence is in the nominative case.
"What we call nature is a certain self-regulated motion or change." Here the subject of is, etc., is what we call nature; but of this, we is the subject, and what is the direct object of the verb call, so is in the objective case.
Another way.
Some prefer another method of treatment. As shown by the following sentences, what is equivalent to that which:—
It has been said that "common souls pay with what they do, nobler souls with that which they are."—Emerson.
That which is pleasant often appears under the name of evil; and what is disagreeable to nature is called good and virtuous.—Burke.
Hence some take what as a double relative, and parse that in the first clause, and which in the second clause; that is, "common souls pay with that [singular, object of with] which [singular, object of do] they do."
INDEFINITE RELATIVES.
List and examples.