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