Traditional grammar defines the object in a sentence as the entity that is acted upon by the subject. There is thus a primary distinction between subjects and objects that is understood in terms of the action expressed by the verb, e.g. Tom studies grammar--Tom is the subject and grammar is the object. Traditional sentence structure divides the simple sentence into a subject and a predicate, whereby the object is taken to be part of the predicate. Many modern theories of grammar (e.g. dependency grammars), in contrast, take the object to be a verb argument like the subject, the difference between them being mainly just their prominence; the subject is ranked higher than the object and is thus more prominent.
The main verb in a clause determines whether and what objects are present. Transitive verbs require the presence of an object, whereas intransitive verbs block the appearance of an object. The term complement overlaps in meaning with object: all objects are complements, but not vice versa. The objects that verbs do and do not take is explored in detail in valency theory.
Two object types are acknowledged in grammatical typology: direct and indirect. These object types are illustrated in the following table:
|Direct object||Entity acted upon||Sam fed the dogs.|
|Indirect object||Entity indirectly affected by the action||She sent him a present.|
The descriptions "entity acted upon" and "entity indirectly affected by the action" are merely loose orientation points. Beyond basic examples such as those provided in the table, these orientation points are not much help when the goal is to determine whether a given object should be viewed as direct or indirect. One rule of thumb for English, however, is that an indirect object is not present unless a direct object is also present, and if both are present, the indirect object precedes the direct object.
The traditional grammar of the English language also accounts for the prepositional object, which refers to the word or phrase introduced by a preposition, e.g. Lucy in the sentence She is waiting for Lucy. However, in linguistic typology the term "object" is reserved for arguments of verbs, and in the case of prepositions (and other adpositions) usually the term prepositional or adpositional complement is used.
The term oblique object is also employed at times, although what exactly is meant varies from author to author. Some understand it to be an umbrella term denoting all objects (direct, indirect, and prepositional), whereas others use the term to denote just a prepositional object.
While the typical object is a pronoun, noun, or noun phrase, objects can also appear as other syntactic categories, as illustrated in the following table:
|Noun (phrase) or pronoun||The girl ate fruit.|
|that-clause||We remembered that we had to bring something.|
|Bare clause||We remembered we had to bring something.|
|for-clause||We were waiting for him to explain.|
|Interrogative clause||They asked what had happened.|
|Free relative clause||I heard what you heard.|
|Gerund (phrase or clause)||He stopped asking questions.|
|to-infinitive||Sam attempted to leave.|
|Cataphoric it||I believe it that she said that.|
A number of criteria can be employed for identifying objects, e.g:
Languages vary significantly with respect to these criteria. The first criterion identifies objects reliably most of the time in English, e.g.
The second criterion is also a reliable criterion for isolating languages such as English, since the relatively strict word order of English usually positions the object after the verb(s) in declarative sentences. The third criterion is less applicable to English, though, since English lacks morphological case, exceptions being the personal pronouns (I/me, we/us, he/him, she/her, they/them). For languages that have case and relatively freer word order, morphological case is the most readily available criterion for identifying objects. In Latin and related languages, direct objects are usually marked with the accusative case, and indirect objects with the dative case. However, object marking may also follow non-syntactic rules, such as animacy. In Spanish, for example, human objects have to be marked by the preposition a (as an example of differential object marking).
Verbs can be classified according to the number and/or type of objects that they do or do not take. The following table provides an overview of some of the various verb classes:
|Transitive verbs||Number of objects||Examples|
|Monotransitive||One object||I fed the dog.|
|Ditransitive||Two objects||You lent me a lawnmower.|
|Tritransitive||Three objects||I'll trade you this bicycle for your binoculars.|
|Intransitive verbs||Semantic role of subject||Examples|
|Unaccusative||Patient||The man stumbled twice, The roof collapsed.|
|Unergative||Agent||He works in the morning, They lie often.|
|Ergative||The submarine sank the freighter.|
|Object deletion||We have already eaten dinner.|
|Ergative||The freighter sank.|
|Object deletion||We have already eaten.|
The distinction drawn here between ergative and object-deletion verbs is based on the role of the subject. The object of a transitive ergative verb is the subject of the corresponding intransitive ergative verb. With object-deletion verbs, in contrast, the subject is consistent regardless of whether an object is or is not present.
Objects are distinguished from subjects in the syntactic trees that represent sentence structure. The subject appears (as high or) higher in the syntactic structure than the object. The following trees of a dependency grammar illustrate the hierarchical positions of subjects and objects:
The subject is in blue, and the object in orange. The subject is consistently a dependent of the finite verb, whereas the object is a dependent of the lowest non-finite verb if such a verb is present.