Learn Italian: Oggi il capo è di cattivo umore. - Today the boss is in a bad mood.

In Italian, states of mind or moods often use the preposition di rather than in. So "in a bad mood" translates to the fixed phrase di cattivo umore, just like "in a good mood" is di buon umore.

What if the boss is a woman? Does il capo change?

Yes! If the boss is female, you will often hear la capo (keeping the noun the same but changing the article to feminine) or the fully feminine noun la capa. Both are understood, though la capa is very common in everyday spoken Italian.

Could I say umore cattivo instead of cattivo umore?

While adjectives normally follow the noun in Italian, common descriptive adjectives like cattivo (bad), buono (good), and bello (beautiful) usually go right before the noun. For this specific expression, it is always cattivo umore.

Why isn't there an article before cattivo umore?

In English we say "in a bad mood," but the Italian expression essere di cattivo umore is a fixed phrase that does not use the indefinite article un. Adding it would sound unnatural to a native speaker.