Defective verb

A defective verb is a verb with an incomplete conjugation. Defective verbs cannot be conjugated in certain tenses or moods.

Examples

*in English: can; ought (there is no infinitive "to can," for example)
*in French: pouvoir; gésir (there is no imperative "peux" or "puisse," for example)
*in Spanish: abolir (this is disputed, but some contend that there is no "él abole," for example)
*in Polish: widać, słychać (the only form of these verbs that exists is infinitive)

Defective verbs in English

In the English language there are few defective verbs. One case is the class of modal auxiliary verbs: "can", "may", "will", "must", and so on. These verbs lack several forms. Most notably, despite all having present indicatives, in Standard English they do not have infinitives (to can, to shall), participles (am canning, am shalling, have musted), imperatives, or present subjunctives. Additionally, the third-person singular present lacks the -s common to other verbs ("he can", not "he cans"; "he ought", not "he oughts"). (The archaic pronoun "thou" takes an inflected verb: "thou canst", "thou wilt".) In place of the missing forms, speakers substitute a non-defective phrase with a similar meaning. to must => "to have to", to can => "to be able to", to should => "to be supposed to". In the present tense "I must wash the window" and "I have to wash the window" are interchangeable, but in the future tense "I will have to wash the window" is the only choice.

Another example is the verb be when used with the preposition to to mean 'visit', as in We have been to Paris several times (= 'We have visited Paris several times'). Be is used this way only in the perfect tense, i.e. as a participle with have. Thus English disallows such forms as We are to Paris right now (= 'We are visiting Paris right now'), We must be to Paris next week (= 'We must go to Paris next week'), etc.

Impersonal verbs in English

Impersonal verbs such as "rain" and "snow" share some characteristics with the defective verbs in that conjugations such as "I rain" or "they snow" are not often found; however, the crucial distinction is that impersonal verbs are "missing" certain forms for semantic reasons - in other words, the forms themselves exist and the verb is capable of being fully conjugated with all its forms (and is therefore not defective) but some forms are unlikely to be found because they appear meaningless. Nevertheless, it is possible to imagine metaphorical or even literal sentences where the "meaningless" forms can be found, e.g.
*I rained on his parade.
*I tried to help by clearing their drive but instead I snowed them in.

Contrast the impersonal verb "rain" (where all the forms exist but look semantically odd) with the defective verb "shall" (where only "I shall" is possible):{| to rain