They seem to think that he is descending.
( They ( ( seem ( to ( think ( that ( he ( is descending ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBP seem) (S (VP (TO to) (VP (VB think) (SBAR (IN that) (S (NP (PRP he)) (VP (VBZ is) (VP (VBG descending))))))))) (. .)))
They think he is going down.
( They ( ( think ( he ( is ( going down ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBP think) (SBAR (S (NP (PRP he)) (VP (VBZ is) (VP (VBG going) (PRT (RP down))))))) (. .)))