"It is corn."
(io) sono, (tu) sei, (lui/lei) è, (noi) siamo, (voi) siete, (loro) sono. Make it a habit to check on the conjugation every time you hover over a verb.
I am unsure when to use the definite article before a noun. Is there a rule that one can memorize?
There are lots of rules in Italian when it comes to the use of Definite Articles, but I believe that for the most part they are needed with some exceptions. I found this helpful, hope you do too. http://italian.about.com/od/grammar/a/italian-definite-articles.htm