Help the detective finish her rather unusual crime scene report by selecting the correct articles.
I entered the room and saw _________________________ half-eaten sandwich on the desk. Suddenly, _________________________ sandwich started moving!
I entered the room and saw a half-eaten sandwich on the desk. Suddenly, the sandwich started moving!
Use "a" or "an" when you mention a singular, countable noun for the first time (it's new information to the reader).
Use "the" when you mention that exact same noun again, because now both you and the reader know exactly which sandwich you are talking about!