It brings an undesirable gap that separates modeling and learning in an SMT system.
researchers have proposed alternative approaches to learning synchronous grammars directly from sentence pairs without word alignments, via generative models (Marcu and Wong, 2002;Cherry and Lin, 2007;Zhang et al., 2008;DeNero et al., 2008;Neubig et al., 2011;Levenberg et al., 2012) or discriminative models (Xiao et al., 2012).