Beyond the modal auxiliaries, "beware" is a fully-fledged defective verb of English: it is used as an imperative ("Beware of the dog") and an infinitive ("I must beware of the dog"), but very rarely or never as a finite verb, especially with inflectional endings (*"bewared", *"bewares"). The word "begone" is similar: any usage other than as an imperative is highly marked. Another defective verb is the archaic "quoth", a past tense which is the only surviving form of the verb "quethe", "to say" (related to "bequeath").