1. A method of identifying treelet translation pairs for use in a machine translation system that translates a source language input into a target language output, the method comprising: accessing a corpus of pairs of aligned, parallel syntactic dependency structures, each pair including a source language dependency structure having nodes that represent lexical items, the nodes being aligned with nodes representing lexical items in a target language dependency structure corresponding to the source language dependency structure in the corpus; for one or more of the source language dependency structures in the corpus, enumerating, using a processor of a computer, individual source nodes and combinations of source nodes connected in the source language dependency structure as possible source treelets identifying lexical items, and corresponding dependencies, in the target language dependency structure corresponding to the source language dependency structure, that are aligned with the enumerated nodes and combinations of connected nodes, as possible target treelets corresponding to the possible source treelets, wherein a plurality of possible source treelets are enumerated for the source language dependency structure; extracting well formed treelet translation pairs from the plurality of possible source treelets and possible target treelets; and storing the treelet translation pairs in a data store.