いい versus よい? When do you use which?

Question

What is the difference between いい and よい？

I notice that usually, いい is most commonly used, but sometimes, I get it wrong and よい is the correct answer. What is the situation to use each and what is the difference?

Thanks for the help.

Answer 1

They are quite the same except that よい sounds a bit more formal or contrived depending on the situation but that's all.

What you may already know is that いい only has a 連体形 and a 終止形 which are いい in both cases. For the others bases you have to use よい.

連用形　→　よく・よかっ; ex: よくない・よかった
未然形　→　よかろう

Some expressions explicitly call for よい but it is rare and most of the time rather archaic usage. (ex: 聞くが良い - You should listen)

Answer 2

(Please see this as an appendix to the first poster's answer.)

1. As the other post says, when they can be used interchangeably, いい sounds more informal/colloquial and よい sounds more formal/literary.

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2. いい doesn't have 未然形, 連用形, and 仮定形:

未然: よかろ(-う)
連用: よかっ(-た)・よく
終止: よい・いい
連体: よい・いい
仮定: よけれ(-ば)
命令: ---

You'd also find archaic/fossilized よかれ-/よから-/よき-/よし in set phrases like:

[良]{よ}かれ[悪]{あ}しかれ (for better or worse)
よかれと思って (out of good intentions) 　
よからぬ (≒ よくない、悪い) e.g. よからぬこと　よからぬ噂
よき友 (≒ いい友達)
古き良き時代 (good old days)
よし！/ よっしゃあ！
(～で)よしとする (settle for ~~)

*よかれ, よから and よき are 命令形, 未然形 and 連体形 of archaic adjective よし, respectively.

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3. You use よい for some (set) phrases:

ほどよい (≒ちょうどいい; moderate; proper)
よりよい (better)
よさ (noun form)
よさそう (+ 様態の助動詞「そう」; sounds good)
住みよい (≒ 住み易い; liveable)

And いい for several (set) phrases:

いいね！
かっこいい
いい男 (≒ イケメン)
いい女 (≒ 美人)
「もういいかい？」--「もういいよ」(when playing hide and seek)
いい人ができた (got a boyfriend/girlfriend)
いい奴
いい仲 (romantic relationship)
いい is often used for set phrases with negative nuance or sarcasm:
いい[気味]{きみ}だ
いい[様]{ざま}だ
いい気になる (carried away)
いい加減な (irresponsible, inaccurate, etc.)
いい歳をして
いい顔をする
いい恥さらしだ
いい迷惑だ
いいご身分だ