contrast.mal-malo
Mal vs malo
Mal = adverb (canta mal, me siento mal); malo = adjective (una mala señal). Before masculine singular nouns malo shortens to mal: un mal día.
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Explication
This is the bien/bueno pattern again, on the negative side. Mal is the adverb — it describes a verb and never changes: canta mal, dormí mal, me siento mal, salió mal. Malo is the adjective — it describes a noun and agrees: una mala señal, un negocio malo, malas noticias.
The trap is the apocope: before a masculine singular noun, malo shortens to mal, so mal día, mal humor, mal momento are the adjective, not the adverb. The give-away is that a noun follows (un mal día), whereas adverbial mal attaches to a verb (lo pasé mal).
With ser and estar the meanings split: estar mal = 'to be unwell' or 'to be wrong/incorrect' (eso está mal); ser malo = 'to be bad quality' or 'to be unkind'. And estar de malas means 'to be in a bad mood' or 'out of luck'.
Exemples
I slept badly; it was a bad night.
Région: global
It's not a bad idea.
Région: global
It's a bad moment: the meeting went badly and on top of that my stomach is upset.