csubj : clausal subject

A clausal subject is a clausal syntactic subject of a clause, i.e., the subject is itself a clause.

Kitabı okumanız gerek . \n You need to read the book (You reading the book is necessary)
csubj(gerek, okumanız)

Kitabı okumak gerek . \n You need to read the book (Reading the book is necessary)
csubj(gerek, okumak)

İyi ki okumuşsun . \n Good that you have read (that you read it is good)
csubj(İyi, okumuşsun)