He had forgotten about Adrin.
( He ( ( had ( forgotten ( about Adrin ) ) ) . ) )
(ROOT (S (NP (PRP He)) (VP (VBD had) (VP (VBN forgotten) (PP (IN about) (NP (NNP Adrin))))) (. .)))
He had forgotten that Adrin was going to join them.
( He ( ( had ( forgotten ( that ( Adrin ( was ( going ( to ( join them ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP He)) (VP (VBD had) (VP (VBN forgotten) (SBAR (IN that) (S (NP (NNP Adrin)) (VP (VBD was) (VP (VBG going) (S (VP (TO to) (VP (VB join) (NP (PRP them))))))))))) (. .)))