I did eventually get an LLM to produce what seems to be a correct diagram of a sentence it had never seen, but it took about ten tries. Grammatical analysis seems to have happened correctly every time, but getting to a usable diagram was difficult.

(I know that it's generally rude to include LLM output in HN comments, but in this case I think it's essential supporting material to elevate the discussion of LLM capabilities above "yes it is", "no it isn't".)

First, I just asked ChatGPT (GPT-4o mini, what they offer anonymous free users):

> Please diagram the sentence, "If my mother's road had been cultural, my uncle's sloth squad residue would have compiled a periodic meadow of night."

I just invented that sentence, picking several words with a random number generator, but of course I can't prove to you that I didn't copy it from some book you haven't seen. (For that, you'd have to invent a sentence yourself, or at least provide me with some words to include in it.) I hope you can at least agree that it's a highly unlikely sentence for anyone ever to have uttered before!

After talking to itself for a few paragraphs, it arrived at the following grammatical analysis:

Step 4: Diagramming the sentence

Main Clause:
Subject: "my uncle's sloth squad residue"
Verb: "would have compiled"
Direct Object: "a periodic meadow of night"
"a periodic meadow" (noun phrase)
"of night" (prepositional phrase modifying "meadow")

Conditional Clause:
Subject: "my mother's road"
Verb: "had been" (linking verb)
Predicate Adjective: "cultural"

This is exactly correct. From that it produced the following ASCII art "diagram", which looks like nonsense to me, but maybe it's in some kind of sentence-diagramming formalism I haven't seen:

If
______/______
| |
my mother's had been
road |
cultural

my uncle's sloth squad residue
|
would have compiled
|
a periodic meadow of night
|
of
night

I told it the diagram looked like nonsense, and it produced this one, which looks worse:

If
_______/______
| |
my mother's road had been
| |
cultural (Main Clause)
|
my uncle's sloth squad residue
|
would have compiled
|
a periodic meadow
|
of night

I asked GPT-4 (paid) the same question, and it gave me another exactly correct grammatical analysis in words:

1. **Identify the main clauses**:
- "If my mother's road had been cultural" is a dependent clause (specifically, a conditional clause).
- "my uncle's sloth squad residue would have compiled a periodic meadow of night" is the main clause.

2. **Start with the main clause**:
- Subject: "my uncle's sloth squad residue"
- "my uncle's" modifies "sloth squad," which in turn modifies "residue."
- Predicate: "would have compiled"
- This includes the modal auxiliary verb ("would"), perfect auxiliary ("have"), and main verb ("compiled").
- Direct object: "a periodic meadow of night"
- “a” is an article modifying “meadow,”
- “periodic” modifies “meadow,”
- “of night” is a prepositional phrase modifying “meadow.”

3. **Diagram the dependent clause**:
- Subject: "road"
- Modified by possessive noun phrase “my mother’s”
- Predicate: "had been"
- Complement/Adjective: "cultural"

(I feel like this is definitely "a level that you'd assume to be competitive to a human", in mschuster91's phrase; most native English speakers can't do that.)

But then it came up with the following incorrect diagram:

+---------------------------------------------------+
| Main Clause |
+---------------------------------------------------+
/ | \
/ | \
/ | \
[My uncle's] [sloth squad] [residue]
\ / /
\ / /
\ / /
would have compiled -------------------
\
\
\
[a periodic meadow of night]
/ | \
/ | \
a periodic [of night]

+---------------------+
| Dependent Clause |
+---------------------+
If
|
road
___|_____
| |
my mother's had been cultural

Telling it the diagram was nonsense didn't help.

But my conclusion from this is not that GPT-4 can't analyze the grammatical structure of sentences it had never seen. It's that GPT-4 gets "confused" when it tries to draw ASCII art, like a person trying to tell you how to draw it over the telephone, but not quite as bad. I've seen similarly abominable results when it tried to draw circuit schematics and guitar tablatures as ASCII art. I tried asking it for a graphviz diagram, which was syntactically correct graphviz code but didn't express the sentence structure properly.

I tried simplifying the problem with the prompt, 'Please provide a syntactic tree showing the syntactic derivation of the sentence, "My uncle's sloth squad residue would have compiled a periodic meadow of night," using graphviz syntax and with individual words at the leaf nodes of the tree.' which yielded a slightly incorrect diagram. (I'm still sure nobody has ever uttered this sentence before.) On being told, "This diagram is not quite right.", GPT-4 produced the following diagram, which I believe is correct except for an extra "s" on "uncle" (you can see this in your browser at https://is.gd/Axufjx):

digraph G {
node [shape=plaintext];

S [label="S"];
NP [label="NP"];
VP [label="VP"];

S -> NP;
S -> VP;

// Noun Phrase breakdown
DP_my [label="DP"];
DP_my -> My;

UnclePossessive [label="NP"];
UnclePossessive -> Uncles;
UnclePossessive -> ApostropheS;

NP -> DP_my;
NP -> UncleCompound;

UncleCompound [label="NP"];
SlothSquadResidue[label = "N'"];
SlothSquadResidue-> Sloth;
SlothSquadResidue-> SquadResidue;

SquadResidue[label = "N'"]
SquadResidue-> Squad;
SquadResidue-> Residue;

UncleCompound-> UnclePossessive;
UncleCompound-> SlothSquadResidue;

// Verb Phrase breakdown
ModalWould[label = "Modal"];
ModalWould-> Would;

AuxHave[label = "Aux"];
AuxHave-> Have;

VCompiled[label = "V"];
VCompiled-> Compiled;

ObjectNP[label = "NP"]; // Direct object noun phrase
ArticleA[label = "D"];
ArticleA-> A;

MeadowPhrase[label = "N'"]; // 'Meadow' modified by 'periodic'
AdjPeriodic[label = "Adj"];
AdjPeriodic-> Periodic;

MeadowWord[label = N];
MeadowWord -> Meadow;

MeadowPhrase -> AdjPeriodic;
MeadowPhrase -> MeadowWord;

PPofNight[label = PP]; // Prepositional phrase 'of night'
PrepOf[label=P]
PrepOf-> Of;

NightNounPhrase[label=N]
NightNounPhrase-> Night;

PPofNight -> PrepOf;
PPofNight -> NightNounPhrase;

ObjectNP -> ArticleA;
ObjectNP -> MeadowPhrase;
ObjectNP -> PPofNight;

VP -> ModalWould;
VP -> AuxHave;
VP -> VCompiled
VP -> ObjectNP

}

Claude produced a reasonably serviceable diagram on the first try with that prompt:

digraph SyntacticTree {
node [shape=plaintext];

S -> NP1;
S -> VP;

NP1 -> DET1 [label="Det"];
NP1 -> NP2 [label="NP"];

NP2 -> NP3 [label="NP"];
NP2 -> N1 [label="N"];

NP3 -> N2 [label="N"];
NP3 -> POS [label="Pos"];

VP -> AUX1 [label="Aux"];
VP -> AUX2 [label="Aux"];
VP -> V [label="V"];
VP -> NP4 [label="NP"];

NP4 -> DET2 [label="Det"];
NP4 -> ADJ [label="Adj"];
NP4 -> N3 [label="N"];
NP4 -> PP [label="PP"];

PP -> P [label="P"];
PP -> NP5 [label="NP"];

NP5 -> N4 [label="N"];

DET1 -> "My";
N2 -> "uncle";
POS -> "'s";
N1 -> "sloth";
N1 -> "squad";
N1 -> "residue";
AUX1 -> "would";
AUX2 -> "have";
V -> "compiled";
DET2 -> "a";
ADJ -> "periodic";
N3 -> "meadow";
P -> "of";
N4 -> "night";
}

On being told, I think incorrectly, "This diagram is not quite right.", it produced a worse diagram.

So LLMs didn't perform nearly as well on this task as I thought they would, but they also performed much better than you thought they would.