The omitted information must be predictable and easy to recover for the listener.
It primarily happens at the "left edge" or beginning of a clause. Drop the Low
While it is a grammatical rule in some languages, in others like English, it is often associated with informal registers, such as diary entries , text messages, or casual spoken conversation (e.g., saying "Seen it" instead of "I have seen it"). Topic Drop Across Languages The omitted information must be predictable and easy