All conjugation groups of verbs

Question

I believe most learners of Japanese were taught that the verbs of the language are classified into 3 groups:

Group 1 / Godan Verbs ([五段動詞]【ごだんどうし】​);
Group 2 / Ichidan Verbs ([一段動詞]【いちだんどうし】);
Group 3 / Irregular Verbs;

But apart from those, I've also heard of more groups of verbs other than those 3, like Nidan Verbs or even Yodan Verbs have actually existed.

My question is, do Nidan and Yodan, or even more groups of verbs that I haven’t heard of, still exist in modern Japanese? If so, how do they conjugate? I have been searching for an answer for some time but I don't think there's any.

Also after some researh, I learnt that Nidan verbs have now become Ichidan verbs, and Yodan verbs have become Godan verbs. Is that true?

Answer 1

Nidan and yodan verbs are precursors of ichidan and godan verbs, respectively. If you are totally new to what classical Japanese (文語/古文) is, a good starter is Tofugu's course. In particular, those verb types are listed in this article.

Just as English speakers study Shakespeare in higher education, Japanese students learn the grammar of classical Japanese at high school. Just as thy/thou still exists in modern English, classical Japanese remains in modern Japan. Today, you may encounter the classical grammar in some proverbs, set phrases, samurai dramas, magic spells in fantasy works, and so on. Even a fragmentary knowledge, such as "keri is a past auxiliary like modern ta", will help you enjoy modern games and novels.