Robby zhu posted:
May I ask if it is CGEL (by Huddleston) that you follow/refer to？
I did not consult a grammar book to provide you with the answer above. I simply analyzed the matter. My thinking about free relative clauses has, however, been greatly influenced a 1978 article written by Joan Bresnan and Jane Grimshaw called "The Syntax of Free Relatives in English" (available here).