Hi, Athena. I can tell Yodan is a verb but Ichidan is not. Yodan(予断) means estimating, projecting and forecasting. We usually use this word when in serious situations such as somebody is about to die or the enemy may attack. "Yodan wo yurusanai jyoutai" (予断を許さない状態：よだんをゆるさないじょうたい) is a popular idiom. You can hear this on some serious dramas. Ichidan has some meanings. As a noun, a group, a party and a band or a step. As an adverb, making more and further. You may be said by someone someday. : "You are getting more and more beautiful." or "Your Japanese has been more progressed！" 「あなたは一段と（いちだんと）美しくなっています。」or「あなたの日本語は一段と（いちだんと）進歩しています！」 I wish this could help you♪