I am marking some student work and one of the sentences was
The author is by Katherine Patterson.
What is the term for the error in this sentence?
You could call it a conflation error. It sounds like the student wanted to say either:
but simply conflated the two simple sentences into one:
It’s either an extra word, or the wrong word. There is no special name for it.
The term Katherine Patterson is a predicate nominative. The predicate is equivalent to the subject, author. The correct construction is
The construction by Katherine Patterson is a prepositional phrase that would usually serve as an adjective or adverb.
It can't be an adjective modifying the subject. The author is equivalent to the named person, not characterized by the person.
It can't modify the verb is. The author is not by anyone or anything.
If the subject were book, the phrase could be adverbial, modifying is, defining the source of the book's existence.
If the subject were book, the phrase could, arguably, be adjectival, meaning