a. Jack has never explored but he has already read many books about it.
b. I’m leaving tomorrow but I have not packed my bag yet... I must hurry
c. I have already been to space but I have not walked on the moon yet!
d. Jessica, the movie is a 6 PM. Have you bought the tickets yet ? No sorry I haven’t bought them yet but I have already phoned Matt, he has already bought the tickets for us!