How to use 'ser' and 'estar'

Both ser and estar translate in English as 'to be'. Ser and estar, however, are very different in both meaning and usage. 

First of all it is important to note that both ser and estar are irregular verbs. For example in the present tense their conjugations are ser: soy, eres, es, somos, sois, son ; estar: estoy, estás, está, estamos, estáis, están. Furthermore, both ser and estar can be used as the main verb in a sentence or as auxiliary verbs.

The main difference:

Ser is used to describe permanent or more lasting situations, while estar indicates location and what are considered temporary situations.
Example: Soy de los Estados Unidos.
               Ahora, estoy en Madrid.

When to use SER

If you are looking for a quick way to remember the uses of SER, you can use the acronym DOCTOR, which stands for Description, Occupation, Characteristic, Time, Origin, Relationship.

For a wider understanding here are more uses of the verb ser.

To indicate:

1) Identity of the subject (person, thing or event)
    eg: Éste es el mercado

2) Profession
    eg: Pablo es abogado

3) Qualities unlikely to change (nationality, religion etc.)
    eg: Maria es cubana

4) Description of physical appearance and personality.
    eg: Harry es alto, rubio y simpático

5) Relationships (even after death)
    eg: Tú eres mi amigo

6) Time
    eg: Es la una y media

7) Place and date of an event
    eg: Hoy es el primero de agosto
         Las reuniones son en el salón de conferencias

8) Origin, possession and materials (with preposition 'de')
    eg: mi reloj no es de oro
         La casa es de Katie

9) Quantity and price
    eg: son doce euros

10) Passive voice
      eg: Las playas son admiradas por los turistas

11) Impersonal expressions
     eg: Es urgente llamar a tu casa ahora

When to use ESTAR

For a quick way of remembering when to use the verb estar, you can use the acronym PLACE, which stands for Position, Location, Action, Condition, Emotion.

For a wider undertsnading here are more uses of the verb estar. 

To indicate:

1) Location
   eg: La ciudad de Miami está en la Florida

2) Temporary physical conditions
    eg: Los chicos están enfermos

3) Temporary mental conditions and moods
    eg: Estás deprimada

4) Temporary traits or qualities
    eg: Hombre, estás delgado

5) A temporary situation (with preposition 'de')
    eg: Luisa es ingeniera pero ahora está de camarera en un restaurante muy fino

6) Gerund
    eg: Estamos buscando trabajo