Substitute somebody or anybody in place of Mary and John. Remember that somebody and anybody are singular in their meaning.
Model. Mary and John have opened the door. Somebody has opened the
door.

1. Mary and John are in the room. 2. Do Mary and John have a very fast car? 3. Mary and John are singing a song. 4. Mary and John have closed the window. 5. Are Mary and John knocking at the door? 6. Mary and John are dancing in the hall. 7, Do Mary and John like apples? 8. Mary and John are playing chess. 9. Do Mary and John speak German? 10. Mary and John are listening to the radio.