yoi よい

In Japanese, yoi よい, also spelled yoi 良い, means "good," making it both synonymous and homonymous with the also i-adjective ii 良い.

Basically all the ways ii いい can be used, yoi よい can be used too. Except that using ii いい is more common that yoi よい.

Which is kind of awkward, because the inflections of ii いい, like ikunai いくない, ikatta いかった, are practically never used, while the inflections of yoi よい: yokunai よくない, yokatta よかった, are normally used.

So if you were to say "good" in Japanese, you would say ii いい, not yoi よい. But to say "not good," you would say yokunai よくない, not ikunai いくない.

To make matters worse, both ii and yoi can be spelled with kanji in a dozen different ways: 良い, 好い, 善い, etc. Fortunately, they're normally spelled with hiragana instead.