Malo vs. mal
The Spanish words malo and mal can both be translated as bad, but they are not interchangeable. The fundamental difference is their grammatical function: malo is an adjective that describes nouns, while mal is primarily an adverb that describes actions.
Malo
A1The word malo is an adjective, which means it is used to describe a noun (a person, place, or thing). As an adjective, it must agree in gender and number with the noun it modifies. It changes to malo (masculine singular), mala (feminine singular), malos (masculine plural), and malas (feminine plural). It can mean bad, evil, poor quality, or sick.
A crucial exception: malo shortens to mal when it is placed directly *before* a masculine singular noun.
Este es un libro malo.
(This is a bad book.)
La comida estaba mala.
(The food was bad.)
Son niños malos.
(They are bad children.)
Tengo malas noticias.
(I have bad news.)
Mi hermano se siente malo hoy.
(My brother feels sick today.)
Él es un mal ejemplo para los demás.
(He is a bad example for others.)
Hoy es un mal día para ir a la playa.
(Today is a bad day to go to the beach.)
Tuvo un mal momento en la reunión.
(He had a bad time at the meeting.)
Mal
A1The word mal is an adverb. This means it modifies a verb, describing *how* an action is performed. It answers the question How?. It typically translates to badly, poorly, or wrongly. As an adverb, mal is invariable, meaning it never changes for gender or number. It is also commonly used with the verb sentirse (to feel) to describe a state of being unwell, either physically or emotionally. Occasionally, mal can also be a masculine noun meaning evil or illness.
El equipo jugó mal el sábado.
(The team played badly on Saturday.)
Mi abuela oye mal.
(My grandmother hears poorly.)
No está mal para ser tu primer intento.
(It's not bad for being your first attempt.)
Lo siento, entendí mal la pregunta.
(I'm sorry, I understood the question wrongly.)
Me siento mal, creo que voy a vomitar.
(I feel sick, I think I'm going to vomit.)
Dormí mal anoche por el ruido.
(I slept poorly last night because of the noise.)
La lucha entre el bien y el mal es un tema universal.
(The fight between good and evil is a universal theme.)
Summary
In essence, use malo (and its forms mala, malos, malas) as an adjective to describe *what* something or someone is like (a bad movie, a bad person). Use mal as an adverb to describe *how* an action is performed (to sing badly, to play poorly). The one major exception is to use mal instead of malo when it comes directly before a singular masculine noun (un mal día).