In linguistics, a defective verb is a verb with an incomplete conjugation, or one which cannot be used in some other way as normal verbs can. Defective verbs cannot be conjugated in certain tenses, aspects, or moods.
The most commonly recognized defective verbs in English are auxiliary verbs — the class of preterite-present verbs — can/could, may/might, shall/should, must, ought to, and will/would (would being a later historical development). Though these verbs were not originally defective, in most varieties of English today, they occur only in a modal auxiliary sense. However, unlike normal auxiliary verbs, they are not regularly conjugated in the infinitive mood. Therefore, these defective auxiliaries do not accept each other as objects. Additionally, they do not regularly appear as participles.
For example, can lacks an infinitive, future tense, participle, imperative, and gerund. The missing parts of speech are instead supplied by using the appropriate forms of to be plus able to. So, while I could do it and I was able to do it are equivalent, one cannot say *I will can which is instead expressed as I will be able to. Similarly, must has no true past tense form, this instead being supplied by had the past tense of have, and "to have to" in the infinitive, an example of composite conjugation. The past tense expressing the obligatory aspect of must is expressed as "had to," as in He had to go. "Must have", on the other hand, expresses probability or likelihood in modern English, e.g., "If that's thunder, there must be lightning."
Some verbs are becoming more defective as time goes on; for example, although might is etymologically the past tense (preterite) of may, it is no longer generally used as such (*he might not pass for "he was forbidden to pass"). Similarly, should is no longer used as the past of shall, but with a separate meaning indicating possibility or advice. (However, the use of the preterite form should as a subjunctive form continues, as in If I should go there tomorrow, ..., which contrasts with the indicative form I shall go there tomorrow.) The defective verb ought was etymologically the past tense of owe (the affection he ought his children), but it has since split off, leaving owe as a non-defective verb with its original sense and a regular past tense (owed).
Beyond the modal auxiliaries, beware is a fully-fledged defective verb of English: it is used as an imperative (Beware of the dog) and an infinitive (I must beware of the dog), but very rarely or never as a finite verb, especially with inflectional endings (*bewared, *bewares). Another defective verb is the archaic quoth, a past tense which is the only surviving form of the verb quethe, "to say" (related to bequeath). Certain other verbs are defective only in specific constructions.
Impersonal verbs such as to rain and to snow share some characteristics with the defective verbs in that forms 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 or nonsensical.
Nevertheless, native speakers can typically use and understand metaphorical or even literal sentences where the "meaningless" forms exist, such as I rained on his parade.
Contrast the impersonal verb rain (all the forms of which exist, even if they sometimes look semantically odd) with the defective verb can (only I can and I could are possible). In most cases, a synonym for the defective verb must be used instead (i.e. "to be able to"). (The forms with an asterisk (*) are impossible, at least with respect to the relevant sense of the verb; these phonemes may by coincidence be attested with respect to a homograph [as with "canning" = "the act of preserving and/or packaging in cans"].)
In Arabic, defective verbs are called أفعال جامدة (lit., solid verbs). These verbs do not change tense, nor do they form related nouns. A famous example is the verb ليس laysa, which translates as it is not, although it is not the only auxiliary verb which exhibits this property: some Arabic grammarians argue that دام "daama" (as an auxiliary verb) is also completely defective; those who refute this claim still consider it partially defective. Some other partially defective verbs are "fati'a" and "zaala," which have neither an imperative form nor an infinitive form when used as auxiliary verbs.
At least one Finnish verb lacks the first infinitive form ("non-conjugated" dictionary form of the word). In Finnish, it can be said "kutian helposti" ("I'm sensitive to tickling") but for the verb "kutian" (here conjugated in singular first person, present tense) there is no non-conjugated form. Hypothetically, the first infinitive could be "kudita", but it does not really exist in the language.
Falloir ("to be necessary", only the third-person forms with il exist; the present indicative conjugation (il faut) is certainly the most often used form of a defective verb in French), braire ("to bray", infinitive, present participle and third-person forms only), frire ("to fry", lacks non-compound past forms; speakers paraphrase with equivalent forms of faire frire), clore ("to conclude", lacks an imperfect conjugation, as well as first and second person plural present indicative conjugations), and impersonal weather and similar verbs as in English.
In contemporary German, the verb erkiesen, which means "to choose, to elect" (usually a person for a special task or honour), is only used in the past participle (erkoren) and, quite rarely, the past tense (ich erkor etc.). All other forms, including the infinitive, have long become obsolete and are now unknown and unintelligible to most speakers.
Some verbs have no imperative form, e.g. csuklik (hiccup).
Arsa (says/said), can only be used in past or present tenses. Copula is lacks a future tense, imperative mood, and verbal noun. It has no distinct conditional tense forms, either, but conditional expressions are possible, being expressed with the past tense forms; for example Ba mhaith liom é, which can mean both "I liked it" and "I would like it". The imperative mood is sometimes suppletively created by using the imperative forms of the substantive verb bí. Future tense forms, however, are impossible and can only be expressed through paraphrasing.
말다 (to stop or desist) may only be used in the imperative form or in the hortative form, after an 'action verb + 지' construction. Within this scope, however it can conjugate for different levels of politeness; e.g. 하지마! (Stop that!); 하지마십시오 ('Please, don't do that').
Latin has defective verbs that possess forms only in the perfect system; such verbs have no present tense forms whatsoever. However, these verbs are present in meaning. For example, the first-person form odi and infinitive odisse appear to be the perfect of a verb such as *odo/odio, but in fact have the present-tense meaning "I hate". Similarly, the verb memini, meminisse is conjugated in the perfect:
Instead of "I remembered", "you remembered", etc., these forms signify "I remember", "you remember", etc. Latin defective verbs also possess regularly formed pluperfect forms (with a simple past tense meaning) and future perfect forms (with a simple future tense meaning). Compare deponent verbs, which are passive in form and active in meaning.
The verb coepī, coepisse, which means "to have begun" or "began", is another verb that lacks a present tense system, but it does not have present meaning. Instead, the verb incipiō, incipere is used in the present tense. This is not a case of suppletion, however, because the verb incipere can also be used in the perfect.
The verbs inquit and ait, both meaning "said", cannot be conjugated through all forms.
widać ("it is evident"), słychać ("it is audible"). (The only form of these verbs that exist are the infinitive forms.)
A large number of Portuguese verbs are defective in person, i.e., they lack the proper form for one of the pronouns in some tense. The verb colorir ("to color") has no first-person singular in the present (thus requiring a paraphrase, like estou colorindo ("I am coloring") or the use of another verb of similar meaning, like pintar ("to paint").
Some Russian verbs are defective, in that their first person singular non-past form is lacking: for example, победить ("to win"), убедить ("to convince"), дудеть ("to play the pipe"). These are all verbs whose stem ends in a palatalized alveolar consonant; they are not a closed class, but include in their number neologisms and loanwords such as френдить ("to friend", as on a social network). Where such a verb form would be required, speakers typically substitute a synonymous verb ("Я выиграю"), or use a periphrastic construction involving nominalization and an additional verb ("Я одержу победу").
Many experiential verbs which describe processes that humans cannot generally undergo, such as пригореть ("to be burnt", as food), куститься ("to grow in clusters"), and протекать ("to seep") are ordinarily nonsensical in the first or second person. As these forms rarely appear, they are often described as "defective" in descriptions of Russian grammar. However, this is a semantic constraint rather than a syntactic one; compare the classic nonsensical-but-grammatical sentence Colorless green ideas sleep furiously, or more directly, the English phrase I am raining. First and second person forms of these verbs do see use in metaphor and poetry.
Spanish defective verbs traditionally were some of the verbs which used forms in which the stem ending begins with -i.
- Abolir (The Nueva gramática de la lengua española from the Real Academia (section 4.14d) now conjugates it normally, using abolo / aboles etc.)
- agredir (now mostly normally conjugated)
The rest are not common:
- balbucir found in forms ending in -i, but mostly replaced by balbucear (to stammer).
- garantir (usually replaced by "garantizar", which is regular)
- soler Always used as helping verb, so many forms, although possible, won't make sense.
- usucapir (to acquire property rights through customary use) only in the infinitive in legal texts.
The auxiliary måste (must) lacks the infinitive, except in Swedish dialects spoken in Finland. Also, this verb is unique in having identical present and past tense forms. The verb stinga (sting) lacks the past tense, but the synonym sticka can be used in stead. The verb spritta (startle) lacks a supine form.
Verbs ending on -вісти (розповісти-to tell, perfective and відповісти-to answer, perfective) lack imperative mood forms; imperfective verbs are used instead (for example, відповідай).
Welsh has a handful of defective verbs, a number of which are archaic or literary. Some of the more common ones in everyday use include dylwn (I should/ought) with only imperfect and pluperfect, meddaf (I say) in the present and imperfect and geni (to be born) which has a verb-noun and impersonal forms, e.g. Ganwyd hi (She was born, literally "one bore her").
- Unpaired word – another form of lexical gap
- Girodet, Jean. Dictionnaire du bon français, Bordas, 1981. ISBN 2-04-010580-8,
- Luc Baronian and Elena Kulinich. Paradigm gaps in Whole Word Morphology. Printed in Irregularity in Morphology (and Beyond) (2012). Google Books: 
- http://www.classes.ru/grammar/114.Rosental/13-n/html/unnamed_17.html (Russian)
- Butt, John, A New Reference Grammar to Modern Spanish, 5th Edition, p. 175