5.2 knowledge of sentence structure

　 Knowing which strings of words are grammatical and which are not is part of syntactic knowledge. In addition, native speakers know at least the following:

· Structural ambiguity --- which strings of words have more than one meaning. For example, English speakers know that I like green grapes and pears can be interpreted in two ways.

· Word order --- different arrangements of the same words have different meanings.

· Grammatical relations --- what element relates to what other element directly or indirectly. For example, in The boats are not big enough and We have not enough boats, the word enough is related to different words in the two sentences.

· Recursion --- the repeated use of the same rule(s) to create infinite sentences. You are happy. I know that you are happy. He knows that I know that you are happy. The formation of the last two of these three sentences is based on the same rule. The rule can be applied again and
again.

· Sentence relatedness --- sentences may be structurally variant but semantically related.

· Syntactic categories --- a class of words or phrases that can substitute for one another without loss of grammaticality. Consider these sentences:

　　(21) The child found the knife.
　　(22) A policeman found the knife.
　　(23) The man who just left here found the knife.
　　(24) He found the knife.

All the italicized parts belong to the same syntactic category called noun phrase (NP). The noun phrases in these sentences function as subject. The knife, also a noun phrase, functions as object. The above listed aspects of syntactic knowledge, in addition to knowledge of grammaticality, are implicit knowledge of English speakers. They know these subconsciously. An English-speaking child accumulates all these aspects of knowledge unconsciously. The task of the linguist is to explicate the knowledge, i.e. to spell out the rules explicitly. The rest of this chapter will illustrate how linguists manage to do this difficult job.