Дієслово estar

Estar is the second Spanish "to be". Where ser says what something is, estar says where it is and how it is at the moment: in Madrid, at home, tired, happy, broken, ready. It is irregular in the present, though only slightly.

Four forms carry a written accent, estás, está, estáis, están; estoy and estamos do not. It matters: esta without the accent is "this" (esta casa), está with it is "is".

What estar is for

• Location: El Museo del Prado está en Madrid. ¿Dónde están las llaves? Even a city or a building that will never move takes estar.
• Temporary states and feelings: Estoy cansado. Mi hermana está nerviosa por el examen.
• Health: ¿Cómo estás? - Estoy bien, gracias. El abuelo está enfermo.
• The result of a change: La puerta está abierta. El bar está cerrado los lunes.
• Actions in progress, with a verb in -ando / -iendo: Estoy comiendo. - I'm eating.

How English speakers trip

Because English uses "is" for everything, the instinct is to reach for es whenever you mean "is". The most common result is *El libro es en la mesa; it has to be El libro está en la mesa. A useful test: if the English sentence answers "where?" or "how are you/is it right now?", you want estar.

Two more things that do not translate directly. "How are you?" is ¿Cómo estás?, but "What is he like?" is ¿Cómo es?; the first asks about the moment, the second about the person. And bien and mal go with estar: Está bien ("It's fine / OK"), and *Es bien on its own is a mistake.