Modal verb

In English, modals form a very distinctive class of verbs. They are auxiliary verbs as are be, do, and have, but unlike those three verbs, they are grammatically defective. For example, have → has vs. should → *shoulds and do → did vs. may → *mayed, etc. In clauses that contain two or more verbs, any modal that is present always appears leftmost in the verb catena (chain). Thus, modal verbs are always finite and, in terms of syntactic structure, the root of their containing clause. The following dependency grammar trees illustrate this point:

The verb catenae are in blue. The modal auxiliary in both trees is the root of the entire sentence. The verb that is immediately subordinate to the modal is always an infinitive. The fact that modal auxiliaries in English are necessarily finite means that within the minimal finite clause that contains them, they can never be subordinate to another verb, e.g.,

a. Sam may have done his homework. The modal auxiliary may is the root of the clause.

b. *Sam has may done his homework. Fails because the modal auxiliary may is not the root of the clause.

a. Jim will be helped. The modal auxiliary will is the root of the clause.

b. *Jim is will be helped. Fails because the modal auxiliary will is not the root of the clause.

Such limits in form (tense, etc.) and syntactic distribution of this class of verbs are motivation of the designation defective. Other constructions are frequently used for such a "missing" form in place of a modal, including "be able to" for can, "have to" for must, and "be going to" for shall and will (designating the future). It is of note that in this way, English modal auxiliaries are unlike modal verbs in other closely related languages; see below.

In English, main verbs but not modal verbs always require the auxiliary verb do to form negations and questions, and do can be used with main verbs to form emphatic affirmative statements. (Neither negations nor questions in early modern English used to require do.) Since modal verbs are auxiliary verbs as is do, in questions and negations they appear in the word order the same as do.

Some form of auxiliary "do" occurs in all West Germanic languages except Afrikaans.[5] Its occurrence in the Frisian languages is restricted to Saterland Frisian, where it may be a loan from Low German.[6] In both German and Dutch, the construction has been known since the Middle Ages and is common in dialects, but is considered ungrammatical in the modern standard language.[7] The Duden lists the following three potential uses of tun (to do) in modern German, with only the first being considered standard:[8]

Germanic modal verbs are preterite-present verbs, which means that their present tense has the form of a vocalic preterite. This is the source of the vowel alternation between singular and plural in German, Dutch, and Low Saxon. Because of their preterite origins, modal verbs also lack the suffix (-s in modern English, -t in German, Dutch, Low Saxon and West Frisian, -r in the North Germanic languages, -þ in Gothic) that would normally mark the third person singular form. Afrikaans verbs do not conjugate, and thus Afrikaans non-modal verbs do not have a suffix either:

The main verb that is modified by the modal verb is in the infinitive form and is not preceded by the word to (German: zu, Low Saxon to, Dutch and West Frisian te, Afrikaans om te,). There are verbs that may seem somewhat similar in meaning to modal verbs (e.g. like, want), but the construction with such verbs would be different:

Similarly, in North Germanic languages, the infinitive marker (at in Danish and Faroese, að in Icelandic, å in Norwegian and att in Swedish) is not used for main verbs with modal auxiliaries: Han kan arbejde, han kan arbeta, hann kann arbeiða (he can work). However, there also are some other constructions where the infinitive marker need not be employed, as in Swedish han försöker arbeta (he tries to work).

In English, modal verbs are called defective verbs because of their incomplete conjugation: They have a narrower range of functions than ordinary verbs. For example, most have no infinitive or gerund.

In many Germanic languages, the modal verbs may be used in more functions than in English. In German, for instance, modals can occur as non-finite verbs, which means they can be subordinate to other verbs in verb catenae; they need not appear as the clause root. In Swedish, some (but not all) modal verbs have infinitive forms. This for instance enables catenae containing several modal auxiliaries. The modal verbs are underlined in the following table.

Language	Sentence
English	he must be able to do it
German	er muss das tun können
Swedish	han måste kunna göra det

The Swedish sentence translated word by word would yield the impossible "*he must can do it"; the same goes for the German one, except that German has a different word order in such clauses, yielding "*he must it do can".

Hawaiian Pidgin is a creole language most of whose vocabulary, but not grammar, is drawn from English. As is generally the case with creole languages, it is an isolating language and modality is typically indicated by the use of invariant pre-verbal auxiliaries.[15] The invariance of the modal auxiliaries to person, number, and tense makes them analogous to modal auxiliaries in English. However, as in most creoles the main verbs are also invariant; the auxiliaries are distinguished by their use in combination with (followed by) a main verb.

There are various preverbal modal auxiliaries: Kaen "can", laik "want to", gata "have got to", haeftu "have to", baeta "had better", sapostu "am/is/are supposed to". Unlike in Germanic languages, tense markers are used, albeit infrequently, before modals: Gon kaen kam "is going to be able to come". Waz "was" can indicate past tense before the future/volitional marker gon and the modal sapostu: Ai waz gon lift weits "I was gonna lift weights"; Ai waz sapostu go "I was supposed to go".

Hawaiian, like the Polynesian languages generally, is an isolating language, so its verbal grammar exclusively relies on unconjugated verbs. Thus, as with creoles, there is no real distinction between modal auxiliaries and lexically modal main verbs that are followed by another main verb. Hawaiian has an imperative indicated by e + verb (or in the negative by mai + verb). Some examples of the treatment of modality are as follows:[16]^{: pp. 38–39} Pono conveys obligation/necessity as in He pono i nā kamali'i a pau e maka'ala, "It's right for children all to beware", "All children should/must beware"; ability is conveyed by hiki as in Ua hiki i keia kamali'i ke heluhelu "Has enabled to this child to read", "This child can read".

French, like some other Romance languages, does not have a grammatically distinct class of modal auxiliary verbs; instead, it expresses modality using conjugated verbs followed by infinitives: for example, pouvoir "to be able" (Je peux aller, "I can go"), devoir "to have an obligation" (Je dois aller, "I must go"), and vouloir "to want" (Je veux aller "I want to go").

Modal verbs in Italian form a distinct class (verbi modali or verbi servili).[17] They can be easily recognized by the fact that they are the only group of verbs that does not have a fixed auxiliary verb for forming the perfect, but they can inherit it from the verb they accompany – Italian can have two different auxiliary verbs for forming the perfect, avere ("to have"), and essere ("to be"). There are in total four modal verbs in Italian: potere ("can"), volere ("want"), dovere ("must"), sapere ("to be able to"). Modal verbs in Italian are the only group of verbs allowed to follow this particular behavior. When they do not accompany other verbs, they all use avere ("to have") as a helping verb for forming the perfect.

For example, the helping verb for the perfect of potere ("can") is avere ("have"), as in ho potuto (lit. "I-have been-able","I could"); nevertheless, when used together with a verb that has as auxiliary essere ("be"), potere inherits the auxiliary of the second verb. For example: ho visitato il castello (lit. "I-have visited the castle") / ho potuto visitare il castello (lit. "I-have been-able to-visit the castle","I could visit the castle"); but sono scappato (lit. "I-am escaped", "I have escaped") / sono potuto scappare (lit. "I-am been-able to-escape", "I could escape").

Note that, like in other Romance languages, there is no distinction between an infinitive and a bare infinitive in Italian, hence modal verbs are not the only group of verbs that accompanies an infinitive (where in English instead there would be the form with "to" – see for example Ho preferito scappare ("I have preferred to escape"). Thus, while in English a modal verb can be easily recognized by the sole presence of a bare infinitive, there is no easy way to distinguish the four traditional Italian modal verbs from other verbs, except the fact that the former are the only verbs that do not have a fixed auxiliary verb for the perfect. For this reason some grammars consider also the verbs osare ("to dare to"), preferire ("to prefer to"), desiderare ("to desire to"), solere ("to use to") as modal verbs, despite these always use avere as auxiliary verb for the perfect.[17]

Mandarin Chinese is an isolating language without inflections. As in English, modality can be indicated either lexically, with main verbs such as yào "want" followed by another main verb, or with auxiliary verbs. In Mandarin the auxiliary verbs have six properties that distinguish them from main verbs:[18]^{: pp.173–174}

• They must co-occur with a verb (or an understood verb).
• They cannot be accompanied by aspect markers.
• They cannot be modified by intensifiers such as "very".
• They cannot be nominalized (used in phrases meaning, for example, "one who can")
• They cannot occur before the subject.
• They cannot take a direct object.

The complete list of modal auxiliary verbs[18]^{: pp.182–183} consists of

• three meaning "should",
• four meaning "be able to",
• two meaning "have permission to",
• one meaning "dare",
• one meaning "be willing to",
• four meaning "must" or "ought to", and
• one meaning "will" or "know how to".

Spanish, like French, uses fully conjugated verbs followed by infinitives. For example, poder "to be able" (Puedo andar, "I can walk"), deber "to have an obligation" (Debo andar, "I must walk"), and querer "to want" (Quiero andar "I want to walk").

The correct use of andar in these examples would be reflexive. "Puedo andar" means "I can walk", "Puedo irme" means "I can leave" or "I can take myself off/away". The same applies to the other examples.