いい and よい are the same word after all. The sound of よい（YOI) turned to いい（II), which became dominant in modern Japanese(*). We usually say or even write いい instead of よい. あの人はいい人だ。 もっと勉強したほうがいい 都合がいい いい車 Of course, you can say あの人はよい人だ。もっと勉強したほうがよい。都合がよい。よい車. Those make sense. But I think it is,..... too official. When do we use よい? Let me think... よいこはマネしないでね (Children, be good and don't do this) 良いお年を（Have a happy new year) We don't say いいこはマネしないでね nor いいお年を. They are fixed phrases. However, when you negate it, you should say よくない, not いくない. いい車、買ったね。You bought a good car. いやー、そんなによくないよ。（Thank you for your compliment but) oh, not much good. でもよく(often)ドライブとか行くんでしょ。 but you often take a drive on it. * いい(II)　is said to be a Tokyo dialect, on which modern Japanese was established.