Defective Verbs in English language are – can, may, must, ought, should.
Common characteristics of these verbs:
1. They have not all verb forms
2 .Do not take s in the third person of the present tense singular
I can | I may | I must | I ought | I should |
he can | he may | he must | he ought | he should |
3. They form their interrogative forms by inversion
can I? | may I ? | must I ? | ought I ? | should I ? |
can he ? | may he ? | must he ? | ought he ? | should he ? |
4. They form their negative forms by adding not:
I cannot | I may not | I must not | I ought not | I should not |
he cannot | he may not | he must not | he ought not | he should not |
5. All these verbs are followed by a bare infinitive except the verb ought which must be followed by an infinitive with to:
I can go now
She must go now
They should go now
I ought to go now
6. They have shortened negative forms:
I can’t | I mightn’t | I mustn’t |
I couldn’t | I oughtn’t | I shouldn’t |