Bueno / malo / bien / mal
bueno - good (adjective)
malo - bad (adjective)
bien - well (adverb)
mal - badly (adverb)
Bueno and malo can only be used with nouns. They must match the noun's gender and number (buenos, mala, etc.)
Bien and mal can only be used with verbs. They define how well or how badly you perform an action.
La comida americana, ¿es buena o es mala? Yo canto muy mal, pero mi primo Pedro canta muy bien.
Progress, Vol. 1
Progress, all volumes
Desirable difficulties
If studying feels effortless, you're probably not learning much. A bit of struggle during practice is a good sign. Difficulty isn't the enemy of learning; it's the mechanism.