Defective verb

A defective verb is a verb with an incomplete conjugation. Some defective verbs cannot be conjugated in certain tenses or moods; others can only be conjugated in the third-person singular, with an impersonal subject.

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 (in the sense of, "it need not happen today")

These verbs lack several forms. Most notably, despite all having present indicatives, they do not have infinitives, imperatives, or present subjunctives (not "to can," but "to be able"; not "James, can," but "James, be able"; not "I ask that we may do it," but "I ask that we be allowed to do it"). Additionally, they generally make no distinction between the third-person singular and the other forms of the present tense ("we ought" or "he ought"; not "he oughts").

Impersonal verbs such as "rain" and "snow" can also be called defective, since it would not make sense to say, "I rain" or "they snow."

A few expressions are defective in that they do not deviate from their infinitive forms, such as:

to try and do (meaning, "to try to do")
to go do (meaning, "to go and do," or "to go to do")
to login (meaning, "to log in")

One might say, "I'll try and find out," but never, "I tried and find/found out." Similarly, one might write, "I login every day," but never, "He logsin/logins every day." Many consider these expressions to be solecisms, to be avoided.