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.
- Beware is only used in the imperative and infinitive (Beware of the dog/ Let the buyer beware). It has no tense or aspect. You can't say eg * He never bewares of the dog. The same is true of mind, with the same meaning of beware. Imperative (Mind you don't drop it!) and infinitive (The notice says to mind the gap) forms exist but no finite or participle forms - you can't say eg *I minded the gap. / Don't worry - I'm minding not to drop it.
- 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.
Lewis, M. The English Verb, Cengage (My all time favourite book on verb forms in English)