Hi !!
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.
BE is a linking verb which shows "state of being"
it's the present because we are between the past (when she worked hard) and the next day because she is presenting her project tomorrow.
So we are "today"...
hope this helps ☺☺☺