One of the important parameters in UG is pro-drop parameter. A pro-drop language is a language in which certain classes of pronouns may be omitted when they are in some sense pragmatically inferable. The phenomenon of "pronoun-dropping" is also commonly referred to in linguistics as zero or null anaphora.
In everyday speech there are often instances when who or what is being referred to can be inferred from context. Proponents of the term "pro-drop" take the view that pronouns which in other languages would have those referents can be omitted, or be phonologically null. Among major languages, what might be called a pro-drop language is Japanese (featuring pronoun deletion not only for subjects, but for practically all grammatical contexts). Chinese, Slavic languages, and American Sign Language also exhibit frequent pro-drop features.
Some languages might be considered only partially pro-drop in that they allow deletion of the subject pronoun. These null subject languages include many Romance languages such as Spanish, Italian, Occitan, Catalan, Portuguese, and Romanian (French is the most notable exception), as well as all the Balto-Slavic languages.