The verb estar (conjugation) means "to be". It is irregular in the present tense.

Estar is used to signify the location of an object:
El perro está al lado del gato.
The dog is next to the cat.

Estar + past participle is used to indicate a resultant condition:
Estoy cansado.
I am tired.

Estar is used to express a changeable state or condition:
María está enferma.
María is sick.

Present tense sentences with estoy
Present tense sentences with estás
Present tense sentences with está
Present tense sentences with estamos
Present tense sentences with estan

Estar is an auxiliary when it forms the progressive tenses, as:

Estoy escribiendo (I am writing).

Estaba escribiendo (he was writing).

The verb ser is often confused with the verb estar. Many teachers state the following rule:

Ser denotes a permanent state.

Estar denotes a temporary state.

This rule should not be adopted because it is often incorrect. The following simple rules will enable the student to use the verb estar correctly.

Estar is used to denote:

1. State in locality, viz., to be in a place, as:

• Estoy aquí.
  I am here.
• Londres está en Inglaterra.
  London is in England.

2. A condition, as:

• Estoy candado.
  I am tired.
• Está enfermo.
  He is ill.

Note:

Estar must, of course, be always used before Present Participles, as:

• Está activando sus esfuerzos.
  He is making still further efforts.
• Estamos extendiendo nuestras relaciones.
  We are extending our connection.

Note:
A Present Participle can only express a condition, not a quality.