One approach to acquiring new vocabulary that I’ve always found effective is to seek out the 反対語 (hantai-go, antonyms) of words. To prove my point, how about looking at words that relate to “good” and “bad,” which are about as opposite as you can get.
The word for good, 良い (yoi, also read ryō), is often written using only hiragana. However, sticking with the kanji opens the door to many words, since 良 can be used as a descriptive prefix to indicate that anything is good, such as 良馬 (ryōba, a fine horse).
Yoi, or ryō, work equally well with abstract meanings, such as 良心 (ryōshin, literally a good heart, i.e. one’s conscience) as used in expressions like 彼の良心が咎める (kare no ryōshin ga togameru, his conscience pricks him).
You can easily switch 良 to a negative meaning by adding 不 (fu or bu) such as in 不良少年 furyō shōnen, a boy gone bad, i.e., juvenile delinquent.
I occasionally run into the expression 良妻賢母 (ryōsai kenbo, a good wife and a wise mother), once upon a time seen as the ideal to which every woman should aspire — although I fully concede that the term is fast becoming obsolete. But it’s still alright to use the old expression 良薬は口に苦し (ryōyaku wa kuchi ni nigashi, a good medicine is bitter to the taste. In addition to its literal meaning it’s often used figuratively to mean unpleasant advice is to be welcomed.
The word 良好 (ryōkō, favorable or satisfactory) doubles up on good, since the second character, 好 can mean both “good” and “to like.” The ideograph is composed of 女 (onna, a woman) holding 子 (ko, a child) — no doubt as reassuring a sight in ancient times as it is today, just as 安 (an) shows a woman under a roof, which indicates peace. Use of 好 can be found in words like 好物 (kōbutsu, favorite) and 好機 (kōki, a good opportunity).
While the verbs 好き (suki) and 好む (konomu) both mean “to like,” the popular dish made with eggs and various condiments called お好み焼き (okonomi–yaki, aka “Japanese pizza”) can be translated as “cooked the way you like”; but the beef dish すき焼き (sukiyaki) probably has a completely different origin.
It seems that in premodern times, meat, particularly beef, was not widely consumed in Japan, and when people first began to eat it, they worried that the flesh of a dead animal would “pollute” a ceramic cooking vessel. So in those days “iron chefs” would wash off a 鋤 (suki, shovel) and use it to cook the beef, leeks, tofu and other ingredients over an outdoor fire.
Another good word for good is 善 (zen), which bears no relation to 禅 (the Zen sect of Buddhism). It can be found in such words as 善行 (zenkō, a good deed), 善意 (zen’i, good intentions), or in expressions such as 彼には善悪の観念がない (kare ni wa, zen-aku no kannen ga nai, he cannot tell right from wrong).
Every Japanese adult is familiar with the four-character aphorism 勧善懲悪 (kanzen chōaku, to reward good and punish evil), which figures in morality plays and the like. Another saying, which has a close parallel in English, is 小人閑居して不善を為す (shōjin kankyo shite, fuzen wo nasu, an idle brain is the devil’s shop). Shōjin is a small person — not to be confused with shōnin, the opposite of 大人 (otona, adult) or kobito, the politically incorrect word for a dwarf. In Confucian doctrine it refers to the “inferior man,” i.e., a small-minded person.
The key word here is kankyo, meaning “idle” or at “leisure,” giving a nuance that closely approximates the well-known English saying, “The devil finds work for idle hands.”
The opposite of good is 悪 (aku or warui, wickedness), which can be found in such words as 悪魔 (akuma, a demon); 悪化 (akka, to worsen or deteriorate); 悪意 (aku’i malice); 悪性 (akusei, malignant); 最悪 (saiaku, the worst); and 悪質商法 (akushitsu shōhō, literally bad-natured business methods, but meaning confidence tricks).
For someone to say, in a light tone of voice 悪いね (warui ne) is idiomatically equivalent to “Are you sure it’s all right? I’m afraid I’m imposing on you.”
To say something is 悪くない (waruku nai) on the other hand, can carry a nuance very similar to the same expression in English, “Not bad.” But when it’s preceded by a pronoun such as 私 (watashi) or 僕 (boku) (I, myself), it’s a way of emphatically asserting one is blameless for some real or perceived misdeed.