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Bueno is the basic way to say "good" in Spanish. As an adjective, it is used similarly to the way the word good is used in English. It means something that is suitable, favorable, or morally proper. For example, you might say "Este libro es bueno," or "This book is good." Since bueno is an adjective, it must agree in gender and number with the noun it's modifying. If the noun is feminine, you would say "buena" (boo-EHN-ah). Add an s to the end if the noun is plural.  If you're confused, just look for the thing that's being described as good. That's the noun the adjective bueno needs to agree with. For example, you might say "eso es una buena señal" to mean "this is a good sign." Since the word señal is feminine, you would use the feminine form of the word bueno. If there were several signs rather than just one, you would say "estas son buenas señales." If you are somewhat familiar with Spanish, you may recognize that bueno is the masculine form of the word. But when it occurs immediately before a masculine noun, you drop the ending.  For example, a single friend might lament "Un buen hombre es difícil de encontrar," which means "A good man is hard to find." Since the word hombre is a masculine noun, bueno is shortened to buen. However, if the word bueno appears after the masculine noun it modifies, you don't shorten it at all. For example, "Es un informe bueno" or "It is a good report."  Bueno may generally appear before or after the noun. For example, "el libro bueno" and "el buen libro" are both correct. " The word muy is an adverb that means "very." You can put it in front of bueno to intensify the adjective. While the word bueno still needs to agree with the noun in gender and number, the word muy never changes. For example: "Este vino es muy bueno" (This wine is very good). If you want to talk about something being current, in order, or acceptable, you might use the word good in English. In Spanish, you would use the word válido.  For example you would say "Mi pasaporte es válido por 10 años" to mean "My passport is good [valid] for 10 years." You may also use válido to say that something is correct or accurate. For example: "Es un texto válido." (It is a valid text.) When someone asks you how you're doing (¿Cómo estás?") you might be tempted to reply "Estoy bueno" or "I'm good." Just as in English, this is grammatically incorrect.  Bueno is an adjective. When you say "estoy bueno," this may be interpreted as though you are saying "I am good looking." If you go around telling people that, they'll think you're egotistical.

Summary:
Say "bueno" (boo-EHN-oh) when something is good. Change the ending to agree in gender and number. Shorten the word to "buen" in front of masculine nouns. Add "muy" (MOO-ee) to say "very good. Use "válido" (VAH-lee-doh) if you mean that something is valid. Avoid "bueno" to describe yourself.