How to Draw Phrase Structure Trees

How to Draw Syntax Trees, Part 3: Type 1 - A sentence is an S

Previously on how to draw syntax trees: Part 2: What do we even mean by a syntax tree?

The first type of tree that we're going to talk about begins with a basic generalization that a sentence (such as "the cat plays piano") consists of a noun phrase (such as "the cat") and a verb phrase (such as "plays piano"). You might have learned in high school English class that these two parts are called a subject and a predicate. A noun phrase, in turn, contains an optional determiner and a noun, while a verb phrase contains a verb and an optional further noun phrase. The category label "determiner" can be replaced with "the", the category label "noun" can be replaced with "cat" or "piano", and the category label "verb" can be replaced with "plays". (Of course, there are so very many words out there that in this last step we only list the relevant ones for a particular sentence.)

In addition to writing them as words, you can also write these generalizations more compactly in what's known as rewrite rules or Phrase Structure Rules or PSRs (S can be rewritten/replaced with NP and VP, NP can be rewritten as optional Det and N, etc) as follows:

S → NP VP
NP → (Det) N
VP → V NP
Det → the
N → cat, piano
V → plays

And finally, you can also write these generalizations as a tree. Notice that if we follow the rewrite rules above in order, we'll produce the tree, from top to bottom. But also, if we read along the bottom branches, we'll get our original sentence in the right order: the cat plays piano.

So far, so good. But you might reasonably be asking at this point why we've chosen the particular phrase structure rules that we did. Apart from traditional grammatical terminology of subject and predicate, why might we want to distinguish between that initial noun phrase and verb phrase? Why not say that a sentence consists of a noun phrase, a verb, and a further noun phrase?

S → NP V NP ??

Let's start with what the point is of grouping certain words together under the same node: it's to represent that a particular set of words can act together as a unit (aka a constituent). For example, "the cat" and "piano" are each constituents, and they're also the same type of constituent, because we can substitute both of them with pronouns ("she plays piano", "he plays it", "the cat plays it", even "it plays piano" depending on how fond you are of cats). If we substitute them for each other ("the piano plays the cat"), we get a sentence that involves a bizarre anthropomorphized piano, but it's merely odd, not totally jumbled like "plays cat piano the".

So if "plays piano" is a constituent apart from "the cat", we'd expect to find it grouped together in ways that we can't find, say, just "the cat plays". Does this actually happen? Well, here's an example.

What does the cat do?
Play piano.

What happens to the piano?
*The cat plays.

So "play piano" sounds pretty good as an answer to a question, whereas "the cat plays" just sounds weird (linguists call this weird-sounding-ness being ungrammatical and mark it with a star: *). And that means that "plays piano" is a constituent separate from "the cat", which we can call a verb phrase. Of course, you also want to talk about the whole sentence as a constituent, so that's where you get S.

Sometimes you also have a sentence within another sentence. Let's look at "YouTube shows that the cat plays piano". We can split this into two smaller sentences, such as "YouTube shows the video" and "the cat plays piano", and we know how to draw both of them. But what about the "that"?

"That" in this context is called a complementizer, because it makes one sentence the complement of another. Or in other words, it lets one sentence complete another. In this case, "the cat plays piano" completes the sentence "YouTube shows…", so "the cat plays piano" is the complement of the main sentence, and it gets introduced by the complementizer "that".

So we know what "that" is doing, but where do we put it in the tree? Well, since it's a thing that goes just outside the S, we could consider it a slight variation on S, and consider it part of a larger constituent called S', pronounced S-prime, which is terminology borrowed from math. And if you put the two sentences together, you get something that looks like this.

The last thing that I want to point out about this approach is that you can branch as many things off a phrasal node as necessary. For example, "the light blue cat of great talent" is an NP just like "the cat", but it also contains two adjectives (Adj, inside AdjP), a degree word (Deg), a preposition (P, inside PP), and an additional noun.

Note that attaching "light" to the NP would mean that the cat was light (in which case "light" would be an adjective, just like "blue" is), whereas attaching "light" to the AdjP, like we've done here, means that the shade of blue is light (and "light" is considered a degree word, which is the term linguists use for words that modify adjectives or prepositions).

At any rate, you can definitely do more complex things with this type of phrase structure rule, sentence=S approach, but this gives you a sense of the basics. I'm not getting into transformations/movement, but here's an overview of it from a basically-PSR approach. This type of structure is the oldest in the history of generative syntax, so many courses start with it because it's also fairly simple. The simplicity is nice, because we have less "stuff" to keep track of, but we'll see coming up next how the simplicity also means that we can't express as much detail as we want to sometimes.

This is part 3 of a series on how to draw syntax trees and how to make sense of the various online resources for doing so. Other posts in  the series are below:

1. So, you asked the internet how to draw syntax trees. Here's why you're confused.
2. What do we even mean by a syntax tree?
3. Type 1: A sentence is an S
4. Type 2: A sentence is an IP
5. Type 3: A sentence is a TP
6. Reconciling theories and final notes
7. Other resources and topics
8. A step-by-step guide to drawing trees, with gifs

spencerelithe.blogspot.com

Source: https://allthingslinguistic.com/post/100617668093/how-to-draw-syntax-trees-part-3-type-1-a

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