A key challenge of text difficulty evaluation is that linguistic difficulty arises from both vocabulary and grammar (Richards and Schmidt, 2013).
most existing tools either do not sufficiently take the impact of grammatical difficulty into account (Smith III et al., 2014;Sheehan et al., 2014), or use traditional syntactic features, which differ from what language students actually learn, to estimate grammatical complexity (Schwarm and Ostendorf, 2005;Heilman et al., 2008;François and Fairon, 2012).