Grammatical relation is a part of linguistics that studies the relationships of elements within a clause, phrase, or sentence from a grammatical point of view. The primary means of studying this is through the relationships of the subject, objects, adjuncts, and complements. These relationships then determine the grammatical cases and categories of the words contained with the sentence and thus help to determine the syntax of the sentence.
Linguists have used grammatical relation theory to determine the idea of relational grammar and also a further development called arc pair grammar. Both are counter-theories to Noam Chomsky's ideas of transformational grammar, which looks at the deeper meanings of sentence structure. In relational grammar, the basic grammatical relationships of a language determine the later development of syntactic relationships. In other words, all conceptual notions are born out of function and not vice versa.
The grammatical relation the subject has with the verb and the object determines the structure. This makes the subject the most important part of the sentence. In the sentence "Bob hit Jim with a cream pie," Bob is the subject and determines the form of the rest of the sentence.
Objects are a thing or person that relates to the subject. There can be no objects, one object, or multiple objects within one clause or sentence. In the sentence "Bob hit Jim with a cream pie," Jim is the first object and the cream pie is the second object. There are three types of object: direct, indirect, and prepositional.
An adjunct is a piece of additional information, which can be removed without the sentence losing meaning. For example, "Bob hit Jim" is just as effective as "Bob hit Jim with a cream pie." The complement is a piece of additional information that does need to be included. For example, "Bob throws" has no real meaning unless the reader knows what is thrown; the complement to this sentence would be "a cream pie."
In the field of grammatical relation, the subject is always the subject and the object is always the object. This is the major difference between grammatical relation and syntax. Once the basic functions and forms of all the words in the sentence have been determined, their thematic or syntactic values can be identified.
Differentiating grammatical relation and syntax in this sense is like splitting apart a layer cake. The layer of cake is the function and the layer of cream is the thematic value. While the subject and object are constant, either one of them can be the agent or the patient, or even the instrument in syntactic terms.
The agent is the person or object that does the action. Patients are the ones that receive the action. Instruments are used to do the action by the agent. The following three sentences demonstrate how the grammatical relation of the words determines the syntactic functions:
"Bob threw a cream pie at Jim."
"Bob got hit with a cream pie thrown by Jim."
"Bob was thrown by Jim at the cream pie."
In all three sentences, Bob is the subject and Jim and the cream pie are objects. Syntactically, however, the subject-verb relationship determined different values. This means that Bob is the agent in the first one, the patient in the second one, and the instrument in the third.