5.4.3 Phrase structure rules

　 Speakers of a language know what words can be strung with what other words to form phrases and sentences. In order words, they know the rules that govern the construction of sentences. Phrase structure rules are rules that specify the constituents of syntactic categories. These rules are part of speakers' syntactic knowledge. Such knowledge exists in the mind of speakers. How can linguists manage to describe these rules?

　 The method linguists use is mainly induction. Consider the following sentences:

　　(25) Adults work.
　　(26) The boy works.
　　(27) Poor boys work.
　　(28) The poor boy delivers papers.
　　(29) The poor boy delivers local papers.
　　(30) The poor boy delivers local papers to every household.
　　(31) The poor boy goes to school.
　　(32) He goes to school.
　　(33) The boy can deliver local papers to every household.

All these sentences contain an NP and a VP. We can therefore write a phrase structure rule that can account for this:

　　(i) S → NP VP

Then we look at the constituents of the NP in each of these sentences and write the following rules to represent the different types of NP:

　　(a) NP → N
　　(b) NP → Det N
　　(c) NP → Adj N
　　(d) NP → Det Adj N
　　(e) NP → Pro

We can then collapse all these single rules into one phrase structure rule that accounts for all the noun phrases:

　　(ii) NP → (Det)(Adj)N / Pro

In the same vein, we can look at the constituents of the VP in every sentence.

　　(a) VP → V
　　(b) VP → V NP
　　(c) VP → V NP PP
　　(d) VP → V PP
　　(e) VP → AUX V NP PP

Based on these a general VP rule can be written:

　　(iii) VP → (Aux) V (NP) (PP)

All the prepositional phrases contain a preposition followed by a noun phrase, hence the PP rule:

　　(iv) PP → P NP

All the constituents of the NP within the verb phrase and the prepositional phrases are covered by rule (ii) already.

　 The four rules generalized above represent various aspects of syntactic knowledge. The syntactic categories on the right of the arrow are the immediate constituents of the category on the left. The constituents within brackets are optional, i.e. they may or may not appear. The constituents within brace brackets are alternatives. Each of them can be the constituent of the category, but they do not co-occur, i.e. they do not appear simultaneously as constituents. For example, a noun or a pronoun is an obligatory constituent of NP, but they do not appear in the same single English noun phrase. If constituents co-occur, they appear in the order as specified by the rules. For example, a verb may be followed by a noun phrase and/or prepositional phrase. If the three constituents co-occur, the order is: V NP PP.

　 Phrase structure rules underlie every language. Some of the syntactic categories specified in the rules generalized above seem to be universal. In every language a sentence contains NP and VP. But the order of the constituents varies. In some language VP may precede NP. And the constituents within NP and VP may follow a different order from the English phrase structure rules. In French, N may be followed by Adj within NP, although Det precedes N. Children acquiring their mother tongue and foreign language learners have to work out the phrase structure rules in order to be able to put words into sentences.