( Observer ( ( ( may not ) ( ( notice ( falling ( through ( the ( event horizon ) ) ) ) ) ( unless ( ( the ( black hole ) ) ( ( is small ) ( so ( that ( ( tidal forces ) ( are large ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Observer)) (VP (MD may) (RB not) (VP (VB notice) (S (VP (VBG falling) (PP (IN through) (NP (DT the) (NN event) (NN horizon))))) (SBAR (IN unless) (S (NP (DT the) (JJ black) (NN hole)) (VP (VBZ is) (ADJP (JJ small)) (SBAR (RB so) (IN that) (S (NP (JJ tidal) (NNS forces)) (VP (VBP are) (ADJP (JJ large)))))))))) (. .)))
Observer may not notice falling through the event horizon unless the black hole is small so that tidal forces are  large.
(ROOT (S (NP (NP (DT The) (JJ tidal) (NNS forces)) (PP (IN near) (NP (JJ black) (NNS holes)))) (VP (VBP are) (ADJP (RB so) (JJ great) (SBAR (S (NP (PRP they)) (VP (MD can) (ADVP (RB actually)) (VP (VB tear) (NP (NN matter)) (PP (IN from) (NP (DT a) (NN companion) (NN star))))))))) (. .)))