Structural Ambiguity

Structural ambiguity occurs when a given phrase can be parsed in more than one way. For example "Mary seemed very keen on the boat" could be be taken to mean either:

1. Mary was enthusiastic about the boat
2. Mary was very keen when she was on the boat

               S 
               |
	   +-------+
          NP       VP 
         /         |
        /       +------+
       N        V      AP
       .        .      |
       .        .      +----+--------+
       .        .     Adv  Adj	     PP
       .        .      .    .        |
       .        .      .    .	  +------+
       .        .      .    .	  P     NP
       .        .      .    .     .      |
       .        .      .    .     .   +----+
       .        .      .    .     .  Det   N
       .        .      .    .     .   .    .
      Mary   seemed   very keen  on  the  boat 

In the phrase structure diagram representing the second alternative:

               S 
               |
	   +-------+
          NP       VP 
         /         |
        /       +------+-------------+
       N        V      AP            PP
       .        .      |             |
       .        .      +----+     +------+   
       .        .     Adv  Adj	  P     NP
       .        .      .    .     .      |
       .        .      .    .	  .   +----+
       .        .      .    .	  .  Det   N
       .        .      .    .     .   .    .
      Mary   seemed   very keen  on  the  boat 

Subject/Object Relations

• subject is the NP that is the daughter of S and sister of VP. E.g.
  • Mary caught a fish
  • The boat sank
  • Some people visited the museum
• object is the NP that is the daughter of VP and sister of V. E.g.
  • Mary caught a fish
  • Some people visited the museum
• prepositional object is the NP that is the daughter of a PP and sister of P. E.g.
  • Mary caught a fish with a net
  • Mary spotted the man with the stick

• if a verb is inflected for number, then the inflection will agree with the subject of the sentence. E.g. in the sentence "the man eats some apples", the "~s" affix on the verb indicates third person singular, so must apply to "the man" rather than "some apples". English has relatively little verb inflection though, so this is not as useful in English as in other languages.
• in certain cojoined forms, the subject is what is omitted. For example in the sentence "Phil ate the pizza and was sick", the second phrase "was sick" omits the subject, but we know that it is "Phil", not "the pizza" which was sick.
• In a "tag question", the subject of the sentence will be replaced with a pronoun. E.g. "Bill runs fast, doesn't he?"; "It's raining, isn't it?"

• "The cat (subject) chased the mouse (object)" becomes "the mouse (subject) was chased by the cat (object)"

Why is it important to be able to identify subject/object?

• When studying the grammatical abilities of children, it is important to have some means of categorising the different types of words and sentences they use. For example, young children typically omit the subject when they form sentences
• Similarly, discourse analysis requires terms that can be used to describe what's going on in a series of connected sentences. For example, in the narrative style, the subject and object of a sentence are often references back to items introduced in a previous sentence. For example, "The man brought his dog to the park. He wanted see it run"
• When contrasting modern and historic variants of a language, we can see that changes in the rules have occurred. For example, modern English almost invariably uses SVO, but older texts show that this was not always the case. "For he that wicked counsel giveth is a traitor" would now be written "For he that gives wicked counsel is a traitor". In order to be able to describe these differences, we need concepts of "subject" and "object" etc..

Some Phrase Structure Rules

1. S -> NP Aux VP
   A Sentence can consist of a noun phrase followed by an auxiliary followed by a verb phrase. The auxiliary may be inferred from the verb inflection, e.g. "the boy slept" has no explicit auxiliary, but from the tense of the verb we know that it is "past"
2. NP -> Det N (PP)
   A noun phrase can consist of a determiner, followed by a noun, followed by an optional preposition phrase. E.g.
   • the man (Det N)
   • the man in the moon (Det N PP)
3. VP -> V (NP) (PP)
   A Verb Phrase can consist of a Verb followed by an optional Noun Phrase then an optional Prepositional Phrase. E.g.
   • the boy slept (V)
   • the boy ate some pizza (V NP)
   • the boy hit the nail with the hammer (V NP PP)
   • the boy fell off the wall (V PP)
4. PP -> P NP
   A preposition phrase can consist of a preposition followed by a noun phrase
5. AP -> Adj (PP)
   An adjectival phrase can consist of an adjective followed by an optional preposition phrase
6. NP -> (Det) (AP) N (PP)
   Refines rule 2, the Determiner is optional. E.g.
   • men (N)
   • yellow things in the fridge (AP N PP)
7. VP -> V (NP) (PP) (Adv)
   Refines rule 3, an adverb may be added
8. S' -> Comp S
   A subordinate clause can consist of a COMPLEMENTISER (a word that can introduce an embedded sentence, turning it into a COMPLEMENT, e.g. that, if, whether) and a sentence. Examples:
   • I thought that you were going to school
   • He wondered if it was raining
   • I told you that he thought that you had finished your homework

Useful book readings for this lecture:

• Fromkin+al "An Introduction to Language" pp 137-151