A defective verb is one which does not have a full range of the normal forms.
- used to does not have infinitive, present simple or present participle forms. It is only used in the past simple and (though rarely) past perfect.
- all modal verbs lack infinitive, 3rd person singular present simple and participle forms.
In the case of the modal verbs, where they are defective there is often another, periphrastic verb, which has the same meaning and can replace them when the missing form is needed. For example, can with the meaning of ability can be replaced by be able to.