12.3.2 Clauses and Sentences

Before we move on, we should clarify the status of the S rules in the grammars we just described. S rules are intended to account for entire sentences that stand alone as fundamental units of discourse. However, as we'll see, S can also occur on the right-hand side of grammar rules and hence can be embedded within larger sentences. Clearly then there's more to being an S then just standing alone as a unit of discourse.

What differentiates sentence constructions (i.e., the S rules) from the rest of the grammar is the notion that they are in some sense complete. In this way they correspond to the notion of a clause in traditional grammars, which are often described as forming a complete thought. One way of making this notion of 'complete thought' more precise is to say an S is a node of the parse tree below which the main verb of the S has all of its arguments. We'll define verbal arguments later, but for now let's just see an illustration from the tree for I prefer a morning flight in Fig. 12.4. The verb prefer has two arguments: the subject I and the object a morning flight. One of the arguments appears below the VP node, but the other one, the subject NP, appears only below the S node.