English Translation
Hadn’t we better go now?
Could anyone help me distinguish between “方がいい” and “方がよく”? Their translations seem to be the same? Thank you!
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Yes, いい and よい are equivalent, but in the negative only よく is used.
いい → いくない (my dictionary says this is slang)
よい → よくない / よくありません
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Good question! There are two differences between 良い and いい.
(1) 良い is more formal and often used in writings while いい is more colloquial. For example, the “thumbs-up” icon on social media is translated as いいね (nice!) and we never refer to the icon as 良いね because social media such as Twitter and Facebook is more conversational. Other examples are:
〜した方が良い = 〜した方がいい (had better do X)
今日は天気が良い = 今日は天気がいい (the weather is fine today.)
These are interchangeable and the difference is just about written or oral sentences.
(2) 良い has a wider range of derivative forms than いい. In other words, いい sometimes misses its conjugation words. So we use derivative forms from 良い even in very casual oral dialogues. For example,
良かったですね (That was good for you, or Good to hear that you were in the condition of X.) —> You cannot say いかったですね. 良い + た (past tense) is the only grammatically correct conjugation.
もし天気が良ければ、ピクニックに行こうよ (Let’s go on a picnic if the weather is good.) —> You cannot say もし天気がいければ (i.e. conjugated with 〜れば in an “if” clause).
そんなことをするのは良くないよ (It isn’t ethical to do such a thing.) —> You cannot say いくないよ. 良い + ない (negation) is the only correct combination although いくない is sometimes used in a VERY wacky, childish and girly tone.
Back to the original sentence: “私たちはもう行った方が良くないですか？” Is 良い in this sentence structure interchangeable with いい?
私たちはもう行った方がいくないですか？ is grammatically broken because it’s used in a form of negation and directly conjugated with ない.
私たちはもう行った方がいいんじゃないですか？ is grammatically correct and sounds natural.
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良いのではないか? (Isn’t it better, is it?)
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→ いいのではないか? (Sounds a little more casual)
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→ いいんじゃないか? (Sounds much more casual)
If you alter the original sentence to “私たちはもう行った方が良いですか？” (Should we go now? or Would you like us to go away and leave you alone?),
私たちはもう行った方がいいですか? also works because it’s not a directly conjugated negation.
There is no general rule of thumb. You just need to expose yourself to many conjugation patterns in order to master the difference between 良い and いい.
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Thank you so much!
Your explanation is so detailed (and kind of overwhelming 
).
At least I’ve kept it in mind that " 良い" is more formal and is more appropriate to be applied to negative sentences.
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