Page "Zero (linguistics)" Paragraph 3

* A zero pronoun occurs in some languages.

In the English sentence nobody knows the zero pronoun plays the role of the object of the verb, and in makes no difference it plays the role of the subject.

Likewise, the zero pronoun in the book I am reading plays the role of the relative pronoun that in the book that I am reading.

This is also referred to as PRO.

In pronoun-dropping languages, including null subject languages such as most Romance languages, the zero pronoun is a prominent feature.