Understanding is the key.
( Understanding ( ( is ( the key ) ) . ) )
(ROOT (S (NP (NNP Understanding)) (VP (VBZ is) (NP (DT the) (NN key))) (. .)))
Understanding is the only important aspect
( Understanding ( is ( the ( ( only important ) aspect ) ) ) )
(ROOT (S (NP (NNP Understanding)) (VP (VBZ is) (NP (DT the) (ADJP (RB only) (JJ important)) (NN aspect)))))