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)

Defective verbs in English

In the English language, there are few defective verbs. The only strictly defective verbs are "ought" and the modal auxiliary verbs, including:

• can/could
• may/might
• shall/should
• will/would
• must
• need (e.g. "it need not happen today" - though need is only marginally an auxiliary in that it functions as a full verb too: "it does not need to happen today")

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"), imperatives, or subjunctives. (In some dialects, however, these may be seen; in North Carolina, for example, the phrases "might can", "might could", "might should", and "might would" are common.) Additionally, they make no distinction between the third-person singular and the other forms of the present tense ("he can", not "he cans"; "he ought", not "he oughts") though there are archaic second-person singular forms (such as "thou canst", "thou wilt").

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):