Sarah was given an assignment to write a paper and do a presentation on pollution. She has worked very hard to make her presentation perfect.
This is presenting her project on acid rains tomorrow..
She is nervous.
<em>BE </em><em> is a linking verb which shows "state of being"</em>
<em>it's the present because we are between the past (when she worked hard) and the next day because she is presenting her project </em><u><em>tomorrow</em></u><em>.</em>