Verbo estar, To be

Estar means ‘to be’ when referring to your specific location, (the place where you are) but also works as a descriptor for physical or emotional state.

It can refer to an action happening at the immediate moment.

Its main function is to attribute to the subject a certain quality or state, a function that the verb to be also performs. In general, the verb estar is used when the characteristic that is attributed to the subject is considered by the speaker as the result of an action, transformation or change, real or supposed, or it is considered non-permanent, linked to a spatial situation.

Real Academia Española 

Estar as a location:

I’m at work → Estoy en el trabajo.

She’s with her sister → Ella está con su hermana.

They’re talking → Ellos están hablando.

As a Descriptor:

She’s sick → Ella está enferma.

I’m so tired! → ¡Estoy muy cansado!

He’s sad → Él está triste.

They’re disappointed → Ellos están decepsionados.

Verb forms depending on its basic grammatical time:

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https://www.rae.es/dpd/estar