What is the difference between defective verbs and modal verbs?

Defective verbs are verbs that are not conjugated in all forms or tenses; they may lack certain grammatical forms, such as past or participle forms. In contrast, modal verbs are auxiliary verbs that express necessity, possibility, permission, or ability, such as "can," "may," and "must." While defective verbs might not be fully usable in every context due to their incomplete conjugation, modal verbs are consistently used to modify the meaning of the main verb without changing their forms.