There has been a general rule that transitive verbs are regular and intransitive verbs irregular, at least among verbs that have been with the language since before the beginning. Verbs that came into the language since its importation to the British Isles have had a tendency to be regular regardless of their transitivity. Thus lie, as in lie down is irregular, lie, lay, lain and lay as in lay the book down is regular, lay, laid, laid. Hang retains this formalism only in the sense of hanging a person by the neck until dead. You will find hung used in this context but some will consider it incorrect. On the other hand, others will find hanged in this sense to be pretentious. Language usage is a slippery edged sword.