by Chelsey Cooley Sat Nov 07, 2015 9:19 pm
"has been assigned" would be correct, but it isn't because of tense. The issue here is that "assigned", if it's used as a verb, is an active verb. To use it in the passive form, you need a helper verb, such as "was" or "has been." So, all of these are correct:
The teacher assigned us a fifty-page paper.
A fifty-page paper was assigned by the teacher.
A fifty-page paper has been assigned by the teacher.
The reason the book refers to it as a fragment is that when you see an 'active-looking' verb in a 'passive-looking' context, it actually signals a modifier. Example:
The fifty-page paper assigned by the teacher took me two months to complete.
This is good, because it has the subject 'paper' and the verb 'took'.
The fifty-page paper assigned by the teacher.
This is bad, because it doesn't really have a verb. It just has a subject and a modifier, so it's a fragment.