The models we have discussed so far make no use of syntactic annotation. Both word-based models (Chapter 4) and phrase-based models (Chapter 5) operate on the surface form of words. In this and the next chapter we discuss methods that attempt to improve the quality of statistical machine translation by exploiting syntactic annotation in various ways. This chapter focuses on methods that extend the phrase-based approach, and the next chapter introduces a new framework that is based on the type of tree structures and grammars that are most commonly used in syntactic theory.

Output from phrase-based systems may look like this (translation from French using Europarl data):

While all the content words are translated correctly and the sentences seems coherent at a local level, there are problems with overall grammaticality. There are problems with the placement of verbs and adjectives, which make the output hard to read.