良い, 善い, 好い, 佳い, 吉い, 宜い

In Japanese, 良い, 善い, 好い, 佳い, 吉い and 宜い are different ways to spell with kanji the synonymous i-adjectives ii いい and yoi よい, both of which mean "good."

Because Japanese hates you.

Fortunately these words are normally spelled with hiragana instead, and, although all these different ways technically exist, pretty much only the first few have any real use nowadays. Most of time only 良い is used and that's it.

好い is "good" in the sense the speaker likes it, would prefer it. 善い is "good" as in virtuous, not evil.

The adverbial inflection, yoku 良く, is special in that it can also be spelled as yoku 能く and yoku 克く. The first has a potential nuance: it's "good" you were capable of such feat, while the latter has an endurance nuance: you did "good" overcoming a hardship.