Complex sentence: The group are aided by Earth-2's Flash Jay Garrick before they learn that he is actually a time remnant clone of Hunter Zolomon, who is Zoom, and he steals Barry's Speed Force to cure his terminal illness.
Split into simpler sentences: The group are aided by the Flash of Earth-2's Jay Garrick before they learn that the Flash of Earth-2 is actually a time remnant clone of Hunter Zolomon aka Zoom. Hunter steals Barry's Speed Force to cure his terminal illness.