Bueno vs. buen vs. bien
The Spanish words bueno, buen, and bien can be confusing as they all relate to the concept of good. However, they serve different grammatical functions and are not interchangeable. The key difference is that bueno and buen are adjectives describing nouns (things), while bien is an adverb describing verbs (actions).
Bueno
A1Bueno is an adjective that means good. As an adjective, it describes a noun and must agree with the noun's gender and number (bueno for masculine, buena for feminine, buenos for masculine plural, buenas for feminine plural). It is typically placed after the noun it describes. It can also be used as an interjection like well or okay.
Este libro es bueno.
(This book is good.)
La comida está muy buena.
(The food is very good.)
Ellos son chicos buenos.
(They are good boys.)
Compramos unas manzanas buenas.
(We bought some good apples.)
Es un hombre bueno y trabajador.
(He is a good and hardworking man.)
Bueno, entonces nos vemos mañana.
(Okay, then we'll see each other tomorrow.)
Buen
A2Buen is the shortened, or apocopated, form of the adjective bueno. It also means good, but it is used exclusively when placed *before* a masculine, singular noun.
Hoy es un buen día.
(Today is a good day.)
Él es un buen amigo.
(He is a good friend.)
Tuvimos un buen viaje a México.
(We had a good trip to Mexico.)
Necesitas un buen par de zapatos.
(You need a good pair of shoes.)
Es un buen momento para invertir.
(It is a good time to invest.)
Mi abuelo me dio un buen consejo.
(My grandfather gave me good advice.)
Bien
A1Bien is an adverb that means well. It describes *how* an action is performed, so it modifies a verb, an adjective, or another adverb. Unlike adjectives, bien is invariable; it does not change for gender or number. It is famously used with the verb estar to express a state of being (e.g., to feel well).
Mi hermana cocina muy bien.
(My sister cooks very well.)
No me siento bien hoy.
(I don't feel well today.)
El plan funcionó bien.
(The plan worked well.)
Los niños se portaron bien en la fiesta.
(The children behaved well at the party.)
El trabajo está bien hecho.
(The work is well done.)
¿Cómo estás? Estoy bien, gracias.
(How are you? I am well, thank you.)
Summary
The essential difference is Adjective vs. Adverb.
• Use bueno (or its forms buena/buenos/buenas) to describe a noun (a thing is good). It typically goes after the noun.
• Use buen as a substitute for bueno only when it comes before a masculine singular noun (e.g., un hombre bueno becomes un buen hombre).
• Use bien to describe a verb (an action is done well). It answers the question How?. A simple rule of thumb is that good translates to bueno/buen, and well translates to bien. The main exception is answering How are you? with Estoy bien (I am well).