Learn Spanish (Latin America): ¿Por qué estás de mal humor?

In Spanish, temporary moods or roles are often expressed with the structure estar de, such as estar de buen humor (to be in a good mood) or estar de luto (to be in mourning). While English speakers might be tempted to use tener (like tener hambre), moods specifically use estar de.

Why is it mal and not malo before humor?

Mal is the shortened form of malo. When malo comes directly before a masculine singular noun like humor, it drops the final "o". The exact same rule applies to bueno, which becomes buen in the phrase estar de buen humor.

If I want to say someone is generally a grumpy person, would I change estás to eres?

No, you wouldn't say ser de mal humor. To describe someone who is generally grumpy by nature, you would use an adjective instead, such as ser malhumorado or tener mal carácter. The phrase estar de mal humor is strictly reserved for temporary states.

Does humor always translate to "mood" in Spanish?