I need some information and research URLs about Extraposition in English: 1 2- Extraposition: Obligatory/Optional

Extraposition of Subject and Object in English sentences (S/V/O) may occur, and indeed commonly does occur, when a sentence begins with (S), or has as a mid-element (O), a heavy clause, such as a that-clause or a wh-clause (who, what, when, why, etc) or an infinitival clause that is extraposed from the head of the sentence to the back of the sentence, being replaced by the extrapositional "it."

Research references to the extraposition of subject (S) or object (O) are:

Subject and Object Extraposition in English:
corpus Findings and A Constraint-Based Approach
Jong-Bok Kim
Kyung Hee University

http://mercury.hau.ac.kr/kggc/Publications/SIGG/SIGG15/SIGG15202_JBKim.pdf

A student's introduction to English grammar
By Rodney D. Huddleston, Geoffrey K. Pullum (page 212)

http://books.google.com/books?id=qlxDqB4ldx4C&dq=A+student%27s+introduction+to+English+grammar+By+Rodney+D.+Huddleston,+Geoffrey+K.+Pullum&printsec=frontcover&source=bl&ots=WOGIf3csw4&sig=6j2Ph3P2AKyYZ7VQX3zsyMox9E0&hl=en&ei=t3nnSryEOJK3lAe406CACA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAwQ6AEwAA#v=snippet&q=infinitival&f=false

Additional pages, by Rodney D. Huddleston, Geoffrey K. Pullum:

http://books.google.com/books?id=qlxDqB4ldx4C&dq=A+student%27s+introduction+to+English+grammar+By+Rodney+D.+Huddleston,+Geoffrey+K.+Pullum&printsec=frontcover&source=bl&ots=WOGIf3csw4&sig=6j2Ph3P2AKyYZ7VQX3zsyMox9E0&hl=en&ei=t3nnSryEOJK3lAe406CACA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAwQ6AEwAA#v=onepage&q=extraposition&f=false

Variations in English Object Extraposition [Subject/Object]
Jong-Bok Kim and Ivan A. Sag
Kyung Hee University and Stanford University

http://lingo.stanford.edu/sag/papers/ks-cls-final.pdf

Obligatory extraposition of the subject or object occurs when the sentence makes little or no sense without extraposing the element to the back of the sentence and replacing the extraposed part with the extrapositional "it." For instance, consider Frits Stuurman's amusing example: "I think [that] John left strange." Are we to envision a peculiar John leaving? In this case extraposition of this Object, with replacement by the extrapositional "it," is obligatory: "I think it strange that John left," (page 268).

Optional transposition of S or O occurs when a sentence logically conveys sense with the heavy clause located in a head, mid- or back location. Look at another of Frits Stuurman's examples: "That John left is strange" becomes the extraposed "It is strange that John left," (page 267).

Research resources for Obligatory/Optional Extrapositions are:

Two grammatical models of Modern English: the old and the new from A to Z
By Frits Stuurma

http://books.google.com/books?id=Qd0OAAAAQAAJ&pg=PA267&lpg=PA267&dq=Extraposition:+obligatory/+optional&source=bl&ots=4pkSWcU3B5&sig=xVSbvm3btGdLrQ4sKu3l0HdOn9g&hl=en&ei=qhPnSu2SG4G6lAf2hL2PCA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAsQ6AEwAA#v=onepage&q=Extraposition%3A%20obligatory%2F%20optional&f=false

And also again:

Variations in English Object Extraposition [Obligatory/Optional]
Jong-Bok Kim and Ivan A. Sag
Kyung Hee University and Stanford University
http://lingo.stanford.edu/sag/papers/ks-cls-final.pdf