The most commonly recognized defective verbs in English are auxiliary verbs—the class of preterite-present verbs—"can/could", "may/might", "shall/should", "must", "ought", and "will/would" ("would" being a later historical development). Though these verbs were not originally defective, in most varieties of English today, they occur only in a modal auxiliary sense. However, unlike normal auxiliary verbs, they are not regularly conjugated in the infinitive mood. Therefore, these defective auxiliaries do not accept each other as objects. Additionally, they do not regularly appear as participles.