What is used only to refer to things and not persons. When used as a relative pronoun what means ‘that which’.
- I mean what I say. (= I mean that which I say.)
- What is one man’s meat is another man’s poison.
- Take down what I say.
- What cannot be cured must be endured.
Note that what cannot be used as an ordinary relative pronoun after a noun or pronoun.
- We have shipped everything that you ordered. (NOT We have shipped everything what you ordered.)
As
As is used as a relative pronoun after such, and sometimes after the same.
- He is such a man as I honor.
- My position is the same as yours.
But
After a negative, the word but is used as a relative pronoun in the sense of who…not or which…not.
- There is no rose but has some thorn. (= There is no rose which does not have some thorn.)
Agreement of the relative pronoun and its antecedent
The relative pronoun must be of the same number and person as its antecedent. Hence the verb which follows the relative pronoun must agree with the antecedent in number and person.
- He, who was so weak, could not walk. (Here the verb was agrees with the antecedent he in number and person.)