Learn Italian: Perché stai così? - Why are you like this?

In Italian, the verb stare (to stay/to be) is used to talk about how someone is doing, their health, or their current mood. Just like we ask Come stai? (How are you?), asking Perché stai così? focuses on someone's temporary state or behavior. For this, stare is the right choice over the regular verb for 'to be' (essere).

What is the infinitive form of stai and is it regular?

The infinitive is stare. It is highly irregular in the present tense! Instead of following normal '-are' rules, its forms are sto, stai, sta, stiamo, state, and stanno.

How does così mean 'like this'?

The word così translates to 'thus', 'so', or 'in this way'. English speakers often want to translate 'like this' word-for-word, but in Italian, just using the single word così is the most natural way to express acting or being a certain way.

Why isn't the word tu (you) in this sentence?

In Italian, the verb ending usually tells you who the subject is. Because stai only ever means 'you are' (informal singular), including the subject pronoun tu is not necessary. You would only add it if you wanted to express special emphasis.