ser y estar
© Lingolex - home page http://lingolex.com/spanish.htm

When "ser" is obligatory

Below are the instances when we have to use "ser"


with an adjective to express a permanent or inherent quality
  El azúcar es dulce

sugar is sweet

  mi padre es calvo my father is bald
  mis hermanas son altas my sisters are tall

for occupations and nationality
  Son italianos

they are Italian

  Pablo es español Pablo is Spanish
  Es profesor She is a teacher

with nouns or pronouns
  es un coche

it is a car

  son niñas they are girls
  somos nosotros it's us
  ¿Eres John? Are you John?

with possessions
  es mio

it is mine

  son de Pedro they are Pedro's
  este es mi libro this is my book

with time, dates and numbers
  son las seis

it is 6 o'clock

  dos y tres son cinco two and three are five
  Es el 21 de diciembre it is 21st December
  la temperatura es de 20 grados it is 20 degrees

with events
 

when there is an event or an occasion such as a party we use "ser" even though we may seem to be indicating position which is usually with "estar"

  La fiesta es en casa de Susana The party is in Susana's house

with passive constructions
 

Although it is not as common as in English there is a Spanish equivalent of the English passive.

  fue construido en 1800 it was built in 1800
  el cuadro fue robado The picture was stolen

when speaking about what something is made of
  esos zapatos son de plástico those shoes are (made of) plastic
  es de oro it's made of gold


with "how much / many?"
  ¿cuánto es? How much is it? (asking about the price)
  ¿cuántos son? How many are there?