Argument structure describes how many and what kinds of participants a verb requires — its valency — while thematic roles (theta roles) classify the semantic relationship each participant bears to the event. Core roles include Agent (the intentional doer), Patient (the entity undergoing change), Theme (the entity being moved or located), Experiencer (the entity perceiving or feeling), Goal, Source, and Instrument. Theta theory, within the Principles and Parameters framework, constrains the mapping between thematic roles and syntactic positions through the Theta Criterion: each argument must receive exactly one theta role, and each theta role must be assigned to exactly one argument. Alternations like the dative shift ("gave the book to Mary" / "gave Mary the book") and the causative-inchoative alternation ("broke the vase" / "the vase broke") reveal how argument structure can be reorganized while preserving underlying thematic relations.
Assign thematic roles to every noun phrase in a set of sentences, then test your assignments by paraphrasing — the Agent should still feel like the doer, the Patient like the affected entity. Compare the argument structures of verbs with different valencies (rain = 0, sleep = 1, hit = 2, give = 3) to see how verb meaning constrains participant number. Analyze causative alternations across languages to see which verbs allow them and which resist them.
You have already studied syntactic structure — how sentences are built from phrases according to hierarchical rules — and compositional semantics — how sentence meanings are assembled from word meanings. Argument structure and thematic roles sit at the interface: they describe how the semantic participants in an event are packaged into the syntactic positions that sentences provide.
Start with an observation about verbs: they differ in how many participants they require. "Rain" needs none (it's an event with no participants); "sleep" needs one (the sleeper); "hit" needs two (the hitter and the thing hit); "give" needs three (the giver, the recipient, and the thing given). This is a verb's valency. But valency alone is not fine-grained enough — we also need to know what kind of participant each argument is. This is where thematic roles come in. The doer in "hit" is an Agent (intentional, causing the action); the thing done to is a Patient (undergoing change). "Frighten" also has two arguments, but here the roles differ: "The noise frightened John" has a non-agentive cause (the noise) and an Experiencer who feels the effect (John). Valency tells you how many; thematic roles tell you what kind.
The Theta Criterion, from Chomsky's Principles and Parameters framework, formalizes the constraint that the mapping between roles and arguments must be one-to-one within a clause: each argument gets exactly one role, and each role goes to exactly one argument. This constraint rules out sentences like *"John hit" where the Patient is absent, or hypothetical constructions where two arguments share a single role. The Theta Criterion is why verbs with different argument structures cannot be freely swapped into the same syntactic frame — the frame must match the verb's thematic requirements.
One of the most important results of studying argument structure is discovering that the same verb can appear in multiple frames with different valencies. "The vase broke" (intransitive, Theme only) and "She broke the vase" (transitive, Agent + Theme) use the same verb root but different argument structures. This causative-inchoative alternation, found across hundreds of verbs in many languages, shows that verbs do not have a single fixed argument structure — they have a range of possible structures related by systematic operations. The underlying thematic relation (a Theme undergoing a change of state) is preserved; what varies is whether an Agent is introduced as a cause.
The most persistent misconception to overcome is the equation of "subject" with "Agent." The subject position is defined syntactically (it controls verb agreement, appears before the verb in English declaratives, etc.), but what thematic role it receives depends on the verb. Passive sentences ("The window was broken"), unaccusative verbs ("The ship arrived"), and Experiencer predicates ("The news surprised Maria") all have subjects that are not Agents. Keeping syntactic role (subject, object) and semantic role (Agent, Patient, Experiencer) conceptually separate is the core skill this topic develops.