Sometimes, some authoritative dictionaries give us the adverb below its adjective, but sometimes, for instance penitential, there is no other adverb given.
However, it seems like we need only to add -ly and penitential has its adverb: penitentially. What is the difference between adjectives where the adverb is listed and the ones where it exists, but is not listed?
(ps: I read carefully about the discussion below, actually, it is a question that most Chinese Engligh learners cannot or rarely use stance adverbs or adverb of manner that however, are often used by native speaker. In this case, if a Chinese English learner want to use stance adverbs or adverb of manner, for example, he is mentally ill, comprehensively analyze, how dose s/he know which word has a verb form, and that is why I mentioned dictionaries, which can show us the source)