The past several years have witnessed rapid advances in syntax-based machine translation, which exploits natural language syntax to guide translation. Depending on the type of input, most of these efforts can be divided into two broad categories: (a) string-based systems whose input is a string, which is simultaneously parsed and translated by a synchronous grammar (Wu, 1997; Chiang, 2005; Galley et al., 2006), and (b) tree-based systems whose input is already a parse tree to be directly converted into a target tree or string (Lin, 2004; Ding and Palmer, 2005; Quirk et al., 2005; Liu et al., 2006; Huang.
