an

used before a singular noun that begins with a vowel sound to refer to any member of a group or category, or one particular person or thing

un, una (antes de un sustantivo que comienza con vocal)

• an apple
• an hour

Collocations an idea · an opportunity

• An elephant is a large mammal.

  Un elefante es un mamífero grande.

used before singular countable nouns beginning with the letters 'u' and 'eu', which sound like 'y' in English, to refer to any member of a group or category, or one particular person or thing

un (antes de un sustantivo que comienza con 'u' o 'eu' y suena como 'y')

• an umbrella
• an eucalyptus tree

Collocations an uncle · an unusual situation

• An umbrella is useful in the rain.

  Un paraguas es útil en la lluvia.