Can someone explain to me what 已然形 is? I've seen it brought up being related the hypothetical form like 已然形+ ば but I just can't figure out what it was for. I know its related to classical Japanese but I like to know about these things and Japanese is the first language I've studied so I'm a bit lacking in grammatical knowledge in general. Thank you in advance for anyone who answers
已然形【いぜんけい】 is commonly translated as realis form, as opposed to 未然形 (irrealis form). The literal meaning of 已然 is "already so" or "already happened". This is a term used only in the context of 古語 (archaic Japanese) or 文語 (classical Japanese).
It's the direct ancestor of 仮定形 (hypothetical form) in modern Japanese (also known as 口語, "colloquial" Japanese). 已然形 and 仮定形 conjugate very similarly, and Japanese Wikipedia actually explains them in the same article. So you can think they are just two names of the same conjugation form used in different contexts. According to Wikipedia, historically, the name 已然 was coined first. The name 仮定形 came much later to explain the 口語 version of the Japanese language, which is now called "modern standard Japanese" (see 言文一致).
There is a practical reason to call this form differently depending on whether you are talking about 文語 or 口語; in 文語, this form did not have the hypothetical meaning in the first place (well, basically). For example, in 口語, 書けば of course means "if [someone] writes". However, in 文語, 書けば basically meant "because [someone] writes"! This is one of the trickiest points of classical Japanese when native speakers learn it at high school. See: using 未然形 and 已然形 with conditional ば