Defective verb

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 have been 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 moral obligation. (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).

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