1. a, an
1.1. What are they used for?
1.1.1. They are used to refer to something or someone in the singular. Which function as the two Spanish forms (one, one) In English, as in Spanish, the indefinite article refers to an unspecified thing.
1.2. Its rules and applications:
1.2.1. 1. FOR CLASS MEMBERS.
1.2.1.1. We use a / an with a name or noun to talk about a member of a class. For example
1.2.1.1.1. My uncle is an engineer
1.2.2. 2. TO DESCRIBE.
1.2.2.1. We use a / an when we describe people or things. For example:
1.2.2.1.1. She is a doctor
1.2.3. 3. A / AN NOT USED.
1.2.3.1. We do not use a / an with names or nouns that are written in the plural or are uncountable, because a / an means "one". For example:
1.2.3.1.1. My grand parents are teachers (not...a teachers)
1.3. Main rules
1.3.1. Use “a”, “and” only with singular nouns. For example:
1.3.1.1. a watch
1.3.1.2. a book
1.3.1.3. Exceptions: With the consonant H. For example: an hour
1.3.2. Use “an” with words that start with a vowel sound (a,e,i,o,u). For example:
1.3.2.1. an island
1.3.2.2. an ugly man
1.3.2.3. Exceptions: When the vowel U has de same sound than consonant Y and when the O has de same sound than W. For example: a university