Cluster 0 includes the majority of VBZ and MD (will, won't, can, could), Cluster 1 is mainly composed of past tense VPs (VBD), Cluster 2 has many VBG, and Cluster 3 consists of 86% VBP.
these works and nearly all previous research (Klein and Manning, 2002;Seginer, 2007;Ponvert et al., 2011;Spitkovsky et al., 2013) have focused on unlabeled constituency parsing.