There is a descriptor of a location (i.e: go over there), their is a descriptor relating to a person's ownership of something (their work is overdue). No way are they interchangable. Your university is actually wrong if they allow such a substitution. Much the same as Your, and You're. They are entirely different in meaning, one being a contraction of You Are, and they aren't interchangable either.
Also, according to google and my primary school education, E is only used in arguments by americans, however, I will accept that may have changed in the last 20 years.
Also, according to google and my primary school education, E is only used in arguments by americans, however, I will accept that may have changed in the last 20 years.