いい versus よい? When do you use which?


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.




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)




(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. 

* * *

  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. 

* * *

  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

