This book was first published in 1933. Probably the way English grammar is analyzed has changed since then. Perhaps the technical terms have changed since that time also. Aside from this problems for the present day reader, this book is excellent. In the sentence "I found the cage empty", what is the object of the verb? Jespersen explains that "the cage empty" is the object, where the cage is a primary and empty is a secondary, in his terms the adnex. This book is written as a manual rather than an introduction however, and it would be intimidating to someone who had not studied grammar formally before, like reading a newspaper's bridge column for someone who has never played bridge before.