1-To express an activity going on at the time of speaking.
The teacher is teaching the students
2-To express position or state of something at some point of time
He is sitting on a chair.
We are going to America tomorrow
subject + is/am/are/ + v 1st form + ing + object
I am writing a letter.
She is writing a letter.
They are writing a letter
is/am/are + subject + v1st form + ing + object
Is it raining outside?
Are you listening to music?
Subject + is/am/are + not + v1st form +ing + object
I am not sleeping.
He is not sleeping.
They are not sleeping