From the discussions of the three sentence types, we can conclude that three are generally two major types of mechanisms which are used to build sentence. The Merge operation (or PS rules) creates tree structure by combining categories in a manner consistent with the phrase structure schema of a language. Transformations then modify the tree structures by adding, deleting, or moving an element.
These two types of mechanisms yield two distinct levels of syntactic representations. The first level is called deep structure, which is the basic structure of a sentence formed by the Merge operation. The second level is called surface structure, which results from applying one or more transformations to the deep structure. In cases when no transformation applies, the deep structure and the surface structure of a sentence are the same. Such derivation of the syntactic structure of a sentence may be depicted in Figure 5.19.