6.13 From constituency to tree diagrams

In this section we begin to introduce the formal notation of tree diagrams to represent the structure of phrases and sentences in a way that makes it easier to make specific and testable claims (hypotheses) about them.

Thinking back to Section 6.1, one way of thinking about the goal of syntactic theory is that it’s aiming to account for what languages users know about which sentences are grammatical, and which sentences are ungrammatical.

What constituents do we find inside sentences? Well, we know that a sentence consists of (at least) a subject and a predicate, that subjects are (usually) noun phrases, and predicates are often verb phrases. We might express this as a rule, known as a phrase structure rule.

This rule says that wherever you have an S (a sentence), it is possible for that S to be made up of an NP (noun phrase) followed by a VP (a verb phrase).

Another way to represent the same idea is with a tree diagram, as in Figure 6.1. Tree diagrams can express the same information as phrase structure rules, but can more efficiently express the output of multiple such rules; current syntactic theories are typically expressed in terms of constraints on possible trees, rather than in terms of constraints on phrase structure rules.

What kind of structure might we expect to find inside the NP subject? Here are some NPs—you might think of a sentence in which some or all of them can occur (remember that you can tell if a string in a sentence is a single NP by using a replacement test to try substituting a pronoun).

We could abstract across all of these, and write a general phrase structure rule for NPs, putting parentheses around all optional elements. A subscript “n” indicates that that element can be repeated any number of times.

This can be read as:

An NP can consist of a determiner (optional), followed by a numeral
(optional), followed by any number of adjectives (all optional),
followed by a noun (required), followed by any number of prepositional
phrases (all optional).

We could represent the structure of some of the specific NPs in (2) as in Figure 6.2. Each of these tree diagrams is consistent with the phrase structure rule in (3). The trees represent the pieces that an NP can be made up of: in Figure 6.2 we see a single N (robots) on the right, a determiner (Det) followed by an N (the robots) in the middle, and a determiner followed by a numeral followed by an adjective followed by a noun followed by a prepositional phrase (the six small robots from Boston) on the right.

The phrase structure rule for NPs referred to prepositional phrases (PP). These phrases have fewer possible shapes than NPs do:

Just like we did in (3) for NPs, we can abstract the possibilities for PPs listed in (4) into the single phrase structure rule shown in (5)

We could then expand the “NP” symbol using our phrase structure rule for NPs above. That NP might contain another PP inside it—here we’ve encountered recursion again. Figure 6.3 shows tree structures for some of the PPs in (4).

Now let’s look at some verb phrases (VPs). In the following examples, the VPs are all in [ square brackets ].

How do we know these are VPs? Well, they come after the subject of the sentence (an NP in all these examples). In one case the predicate is a single word arrived—this word is a verb, so the only thing it could be is a verb phrase. All the other sequences in square brackets could be swapped into the same position as arrived, so they must be phrases of the same type. For example, For example, if we replace the predicate arrived in (6c) with [gave the crew orders], we get another grammatical sentence, shown in (7):

This sentence might be a bit pragmatically odd—unless you assume the spaceship is an artificial intelligence that can give orders—but it is grammatical. Another test would be the replacement test for VPs, which involves replacement with do (you can review the replacement test in 6.4 Identifying phrases: Constituency tests).

Based on these tests, we know that a verb by itself (like arrived) can be a VP, and that the object is inside the VP with the preceding verb. In (6) we have sentences with an intransitive VP containing just a verb, transitive VPs with verbs followed by a complement NP or PP, and a ditransitive VP that contains two object NPs.

Many ditransitive verbs in English can also appear with an NP and a PP (and some ditransitive verbs, like put, only allow the NP PP version). The alternation between a ditranstive [NP NP] as in (7) and [NP PP] as in (8) is called the dative alternation.

The VPs we’ve seen in this section can be derived with the following phrase structure rules:

If we looked at a wider range of VPs we’d also find that adverb phrases can go at the beginning or end of VPs, though not typically in the middle. So we’d end up with the following general phrase structure rule for VP:

By putting together the structures we’ve proposed for NPs, PPs, and VPs, we’re now in a position to show some trees for full sentences. Figure 6.4 shows a tree for the sentence [The crew from Mars repaired the spaceship].

There are also some cases where a verb can be followed by an adjective phrase (We are happy.; They seem nice.). There isn’t an easy way to collapse this with our previous phrase structure rule, with the notation we’ve introduced so far, so to account for this we could add a second phrase structure rule for VPs (VP → V AdjP).

We can also formulate phrase structure rules for various modifier phrases: AdvP, AdjP, and NumP.

Adverb phrases consist of an adverb (11a), preceded by an optional degree phrase (11b). You can also sometimes get a PP after an adverb (11c).

This is summarized by the phrase structure rule in (12):

Adjective phrases are similar:

Number phrases are also modified by degree phrases!

The “objects” of adjectives are almost always expressed by PPs—that is, if there’s something in an AdjP that comes after the adjective, it usually can’t be an NP (or a VP), but instead has to be a PP.

Let’s return to phrase structure rules for whole sentences. We’ve already seen that sentences can consist of an NP followed by a VP:

They can also have an auxiliary (modal or non-modal) as in (20a), and can have adverbs at the beginning or end as in (20b).

You can also put negation in a sentence, as in (21)—though negation always requires an auxiliary in English (if there isn’t already an auxiliary, we apply Do-support):

From these we can get a full phrase structure rule with several optional elements, but an obligatory NP subject and an obligatory VP predicate:

Phrase structure rules are useful for describing the sequences that can occur in phrases of different types, but neither these rules nor the trees we’ve seen in this section do more than list the elements that can occur in phrases of different types.

In the remaining sections of this chapter, we’ll explore a theory that limits possible tree structures to a few basic configurations, with the goal of explaining not only how languages can vary, but also explaining limits on the variation seen across human languages.

Check your understanding

Coming soon!

Navigation

If you are following the alternative path through this chapter that interleaves core concepts with tree structures, the previous section was 6.5 Functional categories and the next section is 6.14 Trees: Introducing X-bar theory.

If your instructor has assigned Appendix 1 instead of the second half of this chapter, you should move on to Appendix 1 now.

References