12.3.1 Sentence-Level Constructions

In the small grammar $ \mathcal{L}_{0} $, we provided only one sentence-level construction for declarative sentences like I prefer a morning flight. There are a large number of constructions for English sentences, but four are particularly common and important: declarative structure, imperative structure, yes-no-question structure, and wh-question structure.

Sentences with declarative structure have a subject noun phrase followed by a verb phrase, like “I prefer a morning flight”. Sentences with this structure have a great number of different uses that we will follow up on in Ch. 23. Here are a number of examples from the ATIS domain:

The flight should be eleven a.m. tomorrow

The return flight should leave at around seven p.m.

I'd like to fly the coach discount class

I want a flight from Ontario to Chicago

I plan to leave on July first around six thirty in the evening

Sentences with imperative structure often begin with a verb phrase, and have no subject. They are called imperative because they are almost always used for commands and suggestions; in the ATIS domain they are commands to the system.

Show the lowest fare

Show me the cheapest fare that has lunch

Give me Sunday's flights arriving in Las Vegas from New York City

Show me all flights that depart before ten a.m. and have first class fares

Please list the flights from Charlotte to Long Beach arriving after lunch time. Show me the last flight to leave.

We can model this sentence structure with another rule for the expansion of S:

$$ S\ \to\ \mathit{V P} $$

Sentences with yes-no question structure are often (though not always) used to ask questions (hence the name), and begin with an auxiliary verb, followed by a subject NP, followed by a VP. Here are some examples (note that the third example is not really a question but a command or suggestion; Ch. 23 will discuss the uses of these question forms to perform different pragmatic functions such as asking, requesting, or suggesting.)

Do any of these flights have stops?

Does American's flight eighteen twenty five serve dinner?

Can you give me the same information for United?

Here's the rule:

S $ \rightarrow $ Aux NP VP

The most complex of the sentence-level structures we will examine are the various wh- structures. These are so named because one of their constituents is a wh-phrase, that is, one that includes a wh-word (who, whose, when, where, what, which, how, why). These may be broadly grouped into two classes of sentence-level structures. The wh-subject-question structure is identical to the declarative structure, except that the first noun phrase contains some wh-word.

What airlines fly from Burbank to Denver?

Which flights depart Burbank after noon and arrive in Denver by six p.m?

Whose flights serve breakfast?

Which of these flights have the longest layover in Nashville?

Here is a rule. Exercise 12.10 discusses rules for the constituents that make up the Wh-NP.

$$ S\ \to\ \mathrm{Wh-NP}\ \mathrm{VP} $$

In the wh-non-subject question structure, the wh-phrase is not the subject of the sentence, and so the sentence includes another subject. In these types of sentences the auxiliary appears before the subject NP, just as in the yes-no-question structures. Here is an example followed by a sample rule:

What flights do you have from Burbank to Tacoma Washington?

S $ \rightarrow $ Wh-NP Aux NP VP

Constructions like the wh-non-subject-question contain what are called long-distance dependencies because the Wh-NP what flights is far away from the predicate that it is semantically related to, the main verb have in the VP. In some models of parsing and understanding compatible with the grammar rule above, long-distance dependencies like the relation between flights and have are thought of as a semantic relation. In such models, the job of figuring out that flights is the argument of have is done during semantic interpretation. In other models of parsing, the relationship between flights and have is considered to be a syntactic relation, and the grammar is modified to insert a small marker called a trace or empty category after the verb. We'll return to such empty-category models when we introduce the Penn Treebank on page 21.

There are other sentence-level structures we won't try to model here, like topicalization or other fronting constructions. In topicalization (also treated as a long-distance dependency in the Penn Treebank), a phrase is placed at the beginning of the sentence for discourse purposes.

On Tuesday, I'd like to fly from Detroit to Saint Petersburg