The subject has the grammatical function in a sentence of relating its constituent (a noun phrase) by means of the verb to any other elements present in the sentence, i.e. objects, complements and adverbials.
In the first sentence (which is in the passive voice), the subject is John, while in the second sentence (active voice) it is the police. But when it comes to the representation the action, the actor in both sentences is the police and the goal of the action is John.
In the first sentence, the subject is John, while in the second one it is the the chain. But in the representation of the action or event, the chain plays the same role in both cases, that being the one to which the process is done or happens. This can be seen by considering the fact that the two sentences can be used to describe the same happening. Whenever the first sentence is true, the second one will be true as well, though in the second one it is pictured to have happened without an agent.
The subject was first defined to be the main argument of a proposition. Since then, linguistic theories have been developed to describe languages all over the world. Some theories, such as Systemic Functional Theory, claim all clauses must have a subject no matter what language is being described. Other theories claim there is no such category that is consistent for all languages. In English, though, every clause has a subject.
A subject in English typically matches two types of pattern: agreement and word order. It both agrees with the verb group of its clause and is positioned in certain particular ways. The agreement consists of choosing one of two different forms of the verb (three in the case of the verb be) depending on the number and person of its subject. For instance, if a subject is singular and is a third person, i. e. it is neither the speaker nor the listeners, one chooses the form has of the verb have; otherwise one chooses have. See examples below:
This pattern of agreement is not an absolute rule, because not all verbs have two different forms. Some have only one and never vary in form. E.g.: must, can, will, might, may.
The second pattern of a subject in English is its position in relation to the verb group. When affirming or denying something, one usually places the subject right before the verb group. But when asking a question, one changes the word order by placing the subject after part of the verb group. This means one makes an interrogative clause by changing the declarative word order. Thus an assertion is turned into a question by making a word order change. See the following examples:
Subjects also follow a third pattern. For instance, in English, the pronoun I is usually a subject while me is usually a complement. This system of language that allows us to determine the arguments of a proposition by inflection is called declension and each form is a case of the declining system. In other languages like German, Russian, Latin and Greek, every noun group assumes a case to represent a specific argument of its proposition. The case assumed by subjects is usually (but not always) the one named nominative. Sometimes the subject carries other cases, like the accusative or the dative, depending on the clause structure and the language. Yet other languages, such as Japanese, use a postposition system to determine the arguments of a clause. The classic theorists were very concerned about this language system for both Latin and Greek had declensions, but this is not a concern in modern English grammars anymore, though English has three noun cases (nominative, genitive and an unnamed one):
However, none of theses patterns can be used as a universal pattern of the subject. Not all languages have a subject-verb agreement in verb forms (person and number), noun forms (case, postpositions) or distinctive word orders. And none of these patterns safely determines the subject.
The case system, for instance, is not a universal system that works the same way in all languages. In some languages, when the ergative model is foregrounded, the transitive/intransitive distinction does not affect the cases of the complements. The middle to which some process is done or happens carries the same case no matter if it is the subject or a complement of the verb. In other languages, of which German, Latin and Greek are examples, the subject keeps its case for transitive and intransitive uses of a verb and its quite safe to consider it case-determined.
In languages that lack verb and noun forms for determining the subject, one must determine the subject in terms of word order. For example, in Mainland Scandinavian (Norwegian, Swedish and Danish) the subject occurs either right in front of the tensed verb of a sentence, or follows the verb but precedes the complements.
Finally, in the Topic theory, which is similar but not equivalent to the Theme theory of the School of Prague, the subject is also the topic of a proposition in the default word order. According to this theory, some languages have no means to determine a topic but by making a complement into a subject. So ascribing a passive voice to the verb group is a way to topicalize the said complement: (See also topic-prominent languages.)
Another pattern of the subject is the frequency in which it is ellided (removed/dropped) from the clause. Some languages, like Spanish, Portuguese, Italian, Latin, Greek, Japanese and Mandarin, use this pattern both in assertions and questions. Though most of these languages are rich in verb forms for determining the person and number of the subject, Japanese and Mandarin have no such forms at all. This dropping pattern does not automatically make a language a pro-drop language. For these concerns visit the pro-drop language article. In other languages, like English and French, declarative and interrogative clauses must always have a subject, which should be either a noun group or a clause. This is also true when the clause has no element to be represented by it. This is why verbs like rain must carry a subject such as it, even if nothing is actually being represented by it. In this case it is an expletive and a dummy pronoun. In imperative clauses, though, most languages ellide the subject:
The first sentence means that it was clumsy of Al to sit down (though the manner in which he did so may have been elegant). The second can also mean that the manner in which Al sat down was clumsy (while it may have been highly appropriate to sit down in the first place).
Reflexive pronouns are sometimes subject-oriented. In the following sentence herself is a reflexive pronoun.
This sentence can only mean that Sue assigned the best student to Sue, not that she assigned the best student to the best student.
It is generally assumed that the Noun Phrase occurring with the Verb Phrase, constituting a sentence, is a subject. Copular sentences challenge this view. In a particular class of copular sentences, called "inverse copular sentences", the noun phrase which occurs with the verb phrase plays the role of predicate, occupying the position which is canonically reserved for subjects, and the subject is embedded in the verb phrase (cf. copula). This can be exemplified by pairs of sentences like these pictures of the wall are the cause of the riot (where the preverbal Noun Phrase plays the role of subject and the post-verbal one plays the role of predicate) vs the cause of the riot is these pictures of the wall (where the order is inverse). This has far reaching consequences, affecting for example the theory of expletive subjects and unaccusative verbs (cf. Moro 1997 and Hale - Keyser 2003 and references cited there).