Simple Subject is the main word in the completer subject.
Examples:
That big black cloud on the horizon means rain
(What is the main word in the complete sentence subject? Who or what is doing something in this sentence? The simple subject is cloud)
So, in this sentence:
"Which librarian places orders for a new book"
The main word in this complete sentence is "Librarian"
Hope all of this Helps you with your question
