Impersonal verb
In linguistics, an impersonal verb is one that has no determinate subject. For example, in the sentence ""It rains"", "rain" is an impersonal verb and the pronoun "it" does not refer to anything. In many languages the verb takes a third person singular inflection and often appears with an expletive subject. In the active voice, impersonal verbs can be used to express operation of nature, mental distress, and acts with no reference to the doer. Impersonal verbs are also called weather verbs because they frequently appear in the context of weather description. Also, indefinite pronouns may be called  impersonal, as they refer to an unknown person, like "one" or "someone", and there is overlap between the use of the two. Valency. Impersonal verbs appear only in non-finite forms or with third-person inflection. In the third person, the subject is either implied or a dummy referring to people in general. The term "impersonal" simply means that the verb does not change according to grammatical person. In terms of valency, impersonal verbs are often avalent, as they often lack semantic arguments. In the sentence "It rains", the pronoun "it" is a dummy subject; it is merely a syntactic  placeholder—it has no concrete referent. In many other languages, there would be no subject at all. In Spanish, for example, "It is raining" could be expressed as simply "llueve". Use in meteorological expressions. Temperature expressions ("it is hot"), weather expressions ("it is snowing"), and daylight expressions ("it is dark") tend to lack independent participants with distinct semantic roles. While snow participates in snowing, very few other types of participants can participate, and the participant is indistinguishable from the event itself; this is similar to the phenomenon of cognate objects. In addition, the participating snow is non-specific, and lacks  a clear semantic role. Therefore, assigning the participating snow the role of 'referent' in the default English expression "it is snowing" would seem inappropriate. Instead, linguistics classify the "is snowing" in "it is snowing" as an impersonal verb. Meteorological expressions are often constructed with impersonal verbs in English. However, meteorological expressions are obviously not restricted solely to impersonal verbs, even in English; furthermore, different languages use different strategies for their default meteorological expressions and common idioms. In Palestinian Arabic, "Id-dunya ti-shti" translates to "It (the world) is raining" and uses a non-impersonal verb. "Vreme je sunčano", which  means "the weather is sunny", is a common Serbian construction that uses a (non-impersonal) adverb rather than a verb. Forms. Invisible arguments. When an agent is unspecified, impersonal verbs are also known as zero person construction, or impersonal construction. An implicit argument (an argument that is put forth without stating it directly) is present on a semantic level for both Estonian and Finnish. The Finnic impersonal construction enables an event or state to be described without specifying the identity of the agent (actor). Despite this, the interpretation of the impersonal includes a referent of some sort (dummy). The zero  person is not entirely the same as an impersonal. Finnish: Estonian: There is a lack of an overt nominative subject in these constructions. By-phrase. Some languages require their counterpart to the English by-phrase be present (like Palauan and Indonesian, Austronesian languages). Other languages disallow the presence of a by-phrase. For example, Polish does not allow the use of a by-phrase in its passive. The content in the parenthesis causes the Polish sentence to be ungrammatical as who did the knocking cannot be overtly stated. As such, it might seem like it would be more grammatical  to use impersonal verbs in such cases. Impersonal verbs by language. In some languages such as English, French, German, Dutch and Swedish, an impersonal verb always takes an impersonal pronoun ("it" in English, "il" in French, "es" in German, "het" in Dutch, "det" in Swedish) as its syntactical subject: Occasionally an impersonal verb will allow an object to appear in apposition to the impersonal subject pronoun: Or as an instrumental adjunct: In some other languages (necessarily null subject languages and typically pro-drop languages), such as Portuguese, Spanish, Occitan, Catalan, Italian, Romanian, in Hungarian and all the Slavic languages, an  impersonal verb takes no subject at all, but it is conjugated in the third-person singular, which is much as though it had a third-person, singular subject. Other languages, those which require a subject, may permit an adjunct to assume that role. Indo-European. English. The following sentences illustrate impersonal verbs: The expletive pronoun "it" in these sentences does not denote a clear entity, yet the meaning is clear. In other words, the pronoun "it" has no clear antecedent. English is so strict about requiring a subject that it supplies them for verbs that do not really require them . In sentences (4) and (5), "it" is in the subject position, while the real subject has been moved to the end of the sentence. A simple test can be done to see if the sentence contains an impersonal verb. One checks to see if a given subject pronoun takes an antecedent in the previous clause or sentence, e.g. The two examples may seem similar, but only the pronoun "it" in the first example links with the previous subject. The pronoun "it" in the second example, on the other hand, has no referent. The hill (Bukit Timah) does not rain, "it  rains. This demonstrates that rain" is an impersonal verb. Spanish. There is no equivalent of the dummy subject "it" in Spanish. In Spanish, there are a few true impersonal avalent verbs. Most of them are "atmospheric verbs": Most "impersonal" constructions in Spanish involve using a special verb in third-person defective verb with a direct object as its only argument or use of "impersonal se" (not to be confused with other uses of "se"). There are two main impersonal verbs in Spanish: "haber" (to have) and "hacer" (to do). "Haber" is an irregular verb. When used as an impersonal verb  in the present tense, it has a special conjugation for the third person singular ("hay"). Clauses with the verb "haber" do not have an explicit subject; its only argument is a direct object noun phrase that does not agree with the verb. "Haber" has its 'natural meaning' of "tener" 'to have'. Less frequently, and only in some expressions with a limited number of nouns in singular, the verb "hacer" in the 3rd singular is used as impersonal ("Hacer" is a very common verb meaning 'to do'). Spanish will add the pronoun "se" in front of verbs to form general sentences . Impersonal voice using "se" will use a singular verb since "se" can be replaced by "uno". The passive voice in Spanish has similar characteristics following that of the impersonal "se". It is normally formed by using "se" + the third person singular or plural conjugation of a verb, similar to the impersonal "se". This use of "se" is easily confused with the medial "se". French. The verbs are impersonal in French because they do not take a real personal subject as they do not represent any action, occurrence or state-of-being that can be attributed to a person, place  or a thing. In French, as in English, these impersonal verbs take on the impersonal pronoun - "il" in French. The "il" is a dummy subject and does not refer to anything in particular in this phrase. The most common impersonal form is "il y a", meaning "there is", "there are". Note its other tenses "(il y avait, il y a eu, il y aura, etc.)". French makes a distinction between a dummy subject and an actual subject in clauses with infinitives by the use of a different preposition. The preposition "de" is used with dummy subjects and the preposition "à  is used with real subjects. Compare: German. Impersonal verbs are relatively common in German, often in constructions about a state or process. Common examples include es brennt" ("there is a fire", literally "it burns"), "es zieht" ("there is a draft", literally "it draws") and "es klopft" ("there is a knock at the door", literally "it knocks"), as well as the whimsical "es weihnachtet sehr" ("it is very Christmassy", literally "it is Christmas-ing hard"). Many statements asserting existence also use an impersonal form. Often the equivalent sentences in English start with "there". Celtic languages. The Celtic languages also possess impersonal  verbal forms though their use is usually translated into English by forms such as 'one sees' (Welsh: "gwelir"), 'one did' (Welsh: "gwnaethpwyd"), 'one is' (Irish: "táthar") etc., in which he 'one' is taken to be an empty subject. For weather, personal verbs are used in Celtic languages, e.g. Welsh "Mae hi'n bwrw eira" 'it is snowing'. Verbs meaning existence may also be impersonal. However, sometimes there are intransitive verbs with more or less the same meaning: Latin. Latin has several impersonal verbs, most often seen in the third person singular. The real subject of the sentence will not be in  the nominative case but is most often in the dative or accusative case. These verbs include: Tai-Kadai. Thai. Impersonal verbs in Thai do not allow for an overt grammatical subject. The impersonal verbs occur only with transitive verbs. There is no allowance for the presence of a non-referential subject "man" 'it' in the case frame. In general, it is not allowed in formal speech, such as news reports. However, the presence of non-referential subject "man" can occur in the colloquial form. Subdivision into non-inception and inception subclasses can occur depending on whether the verb may occur  with the path adverb "khin" 'up'. Constructed languages. In the auxiliary language Interlingua, verbs are not conjugated by person. Impersonal verbs take the pronoun "il": In the planned auxiliary language Esperanto, where verbs also are not conjugated for person, impersonal verbs are simply stated with no subject given or implied, even though Esperanto is otherwise "not" a null subject language: In the planned logical language Lojban, impersonal verbs simply have no first argument filled and might not have any arguments filled at all: where "carvi" is a verb meaning "x rains/showers/[precipitates] to x from x" where "x", "x", "x  are numbered core arguments. Comparison to other linguistic classifications. Weather verb. Some linguists consider the impersonal subject of a weather verb to be a dummy pronoun", while others interpret it differently. Adjectives of zero valence are mainly the adjectives referring to weather such as "winding" and "raining" and so on. In some languages such as Mandarin Chinese, weather verbs like "to snow" take no subject or object. Impersonal pronoun. An impersonal pronoun, or dummy pronoun, lacks a reference; in English, the usual example is "it" when used with an impersonal verb. Some sources classify certain uses of "one" (ex. "what  should one say?") or "you" (ex. "you only live once") as "human impersonal pronouns". An impersonal pronoun, when used, serves as an empty placeholder, or "dummy subject", for the sentence. Examples: When the pronoun "one" is used in the numerical sense (rather than as a dummy pronoun), a different pronoun can be used subsequently to referring to the same entity. Generally, it is not ideal to mix the impersonal pronoun one with another pronoun in the same sentence. Null objects. While the concept of impersonal verbs is closely related to phenomenon of null subjects, null objects have to do with  the lack of the obligatory projection of an object position. In French In English Null objects can be understood as implicit anaphoric direct objects, that is, those whose referents can be understood from the prior or ongoing discourse context as well as sufficiently salient in that context not only to be encoded pronominally, but even to be entirely omitted. However, it is not imperative that the referent of the direct object has been referred to explicitly previously in the discourse; it could instead be accessible extra-linguistically due to its salience to the interlocutors. Defective verb. An impersonal verb is  different from a defective verb in that, with an impersonal verb, only one possible syntactical subject is meaningful (either expressed or not), whereas with a defective verb, certain choices of subject might not be grammatically possible, because the verb does not have a complete conjugation. In universal grammar theory. Impersonal verbs can be considered null subject data. They involve a general concern in generative grammar: determining the nature and distribution of phonetically null but syntactically present entities (empty categories). Since, by definition, these entities are absent from the speech signal, it is of interest that language learners still can come  to have information about them. As this phenomenon could not have resulted from sufficient prior experience, it suggests the role of universal grammar.